Q.Two groups are competing for the position on the Board of directors of a corporation. The probabilities that the first and the second groups will win are 0.6 and 0.4 respectively. Further, if the first group wins, the probability of introducing a new product is 0.7 and the corresponding probability is 0.3 if the second group wins. Find the probability that the new product introduced was by the second group.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Conditional Probability
Conditional Probability
Roll a die and ask "what is the chance of an even number?" — that is 3/6. But suppose someone tells you the result is greater than 3. Now you are no longer looking at all six faces, only at {4,5,6}, and two of those (4 and 6) are even, so the probability becomes 2/3. That change — from the probability of A to the probability of A given that B has already occurred — is conditional probability.
The Idea: Shrink the Sample Space
Conditioning on B throws away every outcome where B is false and treats B as the new "whole world." You measure A only against what is still possible.
Think of filtering a table of data: unconditional probability uses every row; conditional probability keeps only the rows where the condition is true.
The Definition
For events A and B with P(B)>0,
P(A∣B)=P(B)P(A∩B).
We divide by P(B) to rescale so that B itself has probability 1; the surviving part of A is the overlap A∩B. Checking the die: P(A∩B)=P({4,6})=62 and P(B)=63, so P(A∣B)=3/62/6=32, matching the intuition.
Rearranging gives the multiplication rule P(A∩B)=P(A∣B)P(B), which is usually the easier way to compute a joint probability when a problem says "given that."
Two Cautions
- P(A∣B) and P(B∣A) are generally not equal; swapping them is the classic mistake. They are linked by Bayes' theorem, P(A∣B)=P(B)P(B∣A)P(A). …
Concept: Conditional Probability (Bayes' Theorem)
We want P(second group wins∣new product introduced).
Let A1 = first group wins, A2 = second group wins, B = new product introduced.
Given:
P(A1)=0.6, P(A2)=0.4,
P(B∣A1)=0.7, P(B∣A2)=0.3.
Step 1: Find P(B) using total probability:
P(B)=P(A1)P(B∣A1)+P(A2)P(B∣A2)=(0.6)(0.7)+(0.4)(0.3)=0.42+0.12=0.54.
Step 2: Apply Bayes' theorem: …
This is a classic Bayes’ theorem problem: we are given the prior probabilities of each group winning and the conditional probabilities of introducing a new product given the winner. The question asks for the reverse conditional probability — that the second group introduced the product, given that a new product was introduced. The answer is 92.
We start with the idea of conditional probability. The problem gives us probabilities in one direction (win → product launch) but asks for the probability in the opposite direction (product launch → which group won). That’s exactly what Bayes’ theorem is built for.
Let’s define the events clearly:
- Let G1 = event that the first group wins.
- Let G2 = event that the second group wins.
- Let N = event that a new product is introduced.
We are told:
- P(G1)=0.6, P(G2)=0.4
- P(N∣G1)=0.7
- P(N∣G2)=0.3
We need P(G2∣N) — the probability that the second group introduced the product, given that a new product was introduced.
- Find the total probability of introducing a new product. The new product can be introduced either when the first group wins or when the second group wins. These are mutually exclusive and exhaustive events. So by the law of total probability:
P(N)=P(N∣G1)P(G1)+P(N∣G2)P(G2)
Substitute:
P(N)=(0.7)(0.6)+(0.3)(0.4)=0.42+0.12=0.54
-
Apply Bayes’ theorem.
Bayes’ theorem says:
P(G2∣N)=P(N)P(N∣G2)P(G2) …
Method: Bayes' Theorem via a probability tree
Use this two-stage "which branch caused the observed second-stage event" pattern — win/lose then launch/no-launch, choose then observe.
Steps
Step 1: Draw the first-stage branches with their priors.
The first stage has outcomes Hi (which group wins) with probabilities P(Hi).
Step 2: Attach the second-stage conditional to each branch.
On each branch write P(E∣Hi), the chance of the observed second-stage event (new product) given that branch.
Step 3: Total probability of the event = sum of the branch products.
P(E)=∑iP(Hi)P(E∣Hi). …
Common Mistakes
Mistake 1: Reporting P(N∣G2)=0.3 as the answer.
Why it's wrong: that is the forward conditional; the question asks P(G2∣N), the reverse. Correct approach: invert with Bayes' theorem.
Mistake 2: Forgetting the first group's term in P(N).
Why it's wrong: a new product can be launched under either winner, so P(N) must sum both branches. Correct approach: P(N)=(0.6)(0.7)+(0.4)(0.3)=0.54. …
Showing the 12 most recent of 45 on this concept.
- KCET 2021Set A-11 markMCQQ.A car manufacturing factory has two plants X and Y. Plant X manufactures 70% of cars and plant Y manufactures 30% of cars. 80% of cars at plant X and 90% of cars at plant Y are rated as standard quality. A car is chosen at random and is found to be of standard quality. The probability that it has come from plant X is (A) 7356 (B) 8456 (C) 8356 (D) 7956
›Reveal solutionSolution
This is a reverse-probability question (effect → cause), so use Bayes' theorem with the total probability of a standard-quality car in the denominator.
Step 1 — Define the events.
- X: the car came from plant X — P(X)=0.70
- Y: the car came from plant Y — P(Y)=0.30
- S: the car is of standard quality
The conditional (likelihood) data given:
P(S∣X)=0.80,P(S∣Y)=0.90
Note X and Y are mutually exclusive and exhaustive (0.7+0.3=1), which is exactly what Bayes' theorem needs.
Step 2 — Why Bayes and not simple conditioning.
We are told the effect (the chosen car is standard) and asked for the probability of the cause (it came from X). That inversion — P(X∣S) from P(S∣X) — is precisely Bayes' theorem:
P(X∣S)=P(X)P(S∣X)+P(Y)P(S∣Y)P(X)P(S∣X)
Step 3 — Compute the numerator.
P(X)P(S∣X)=0.70×0.80=0.56
Step 4 — Compute the denominator (total probability of a standard car).
P(S)=0.70×0.80+0.30×0.90=0.56+0.27=0.83
Step 5 — Divide. …
- COMEDK 2024Set 2024-A1 markMCQQ.P and Q are considering to apply for a job. The probability that P applies for the job is 41. The probability that P applies for the job given that Q applies for the job is 21, and the probability that Q applies for the job given that P applies for the job is 31. Then the probability that P does not apply for the job given that Q does not apply for the job is (A) 54 (B) 87 (C) 65 (D) 1211
›Reveal solutionSolution
We are given conditional probabilities and need to find P(P∣Q). Using the definitions of conditional probability and the law of total probability, we compute P(Q) and P(Q), then apply Bayes' theorem to get 54, which corresponds to option (A).
Concept and intuition:
This is a classic problem of working backwards from conditional probabilities to find a joint probability table. We know P(P), P(P∣Q), and P(Q∣P). From these, we can find P(P∩Q) in two ways, which lets us solve for P(Q). Then we can compute the desired conditional probability P(P∣Q) using the complement rule and the definition of conditional probability.
Step-by-step solution:
- Write down what is given. Let P = event that P applies, Q = event that Q applies. We have:
P(P)=41,P(P∣Q)=21,P(Q∣P)=31.
- Use the definition of conditional probability to express P(P∩Q) in two ways. From P(P∣Q)=P(Q)P(P∩Q), we get
P(P∩Q)=P(P∣Q)⋅P(Q)=21P(Q).
From P(Q∣P)=P(P)P(P∩Q), we get
P(P∩Q)=P(Q∣P)⋅P(P)=31⋅41=121.
- Equate the two expressions for P(P∩Q) to find P(Q).
21P(Q)=121⇒P(Q)=61.
- Find P(Q) and P(P∩Q).
P(Q)=1−P(Q)=1−61=65.
Also, P(P∩Q)=P(P)−P(P∩Q)=41−121=123−121=122=61.
- Find P(P∩Q). Since P∩Q is the complement of P∪Q, we can use:
P(P∩Q)=1−P(P∪Q).
First, P(P∪Q)=P(P)+P(Q)−P(P∩Q)=41+61−121. …
- COMEDK 2025Set 2025-M1 markMCQQ.In an entrance test, there are multiple choice questions. There are four possible answers to each question of which only one is correct. The probability that a student knows the answer to a question is 90%. If he gets the correct answer to a question, then the probability that he was guessing is (A) 3736 (B) 91 (C) 371 (D) 4037
›Reveal solutionSolution
We use Bayes’ theorem to reverse the conditional probability: given that the student answered correctly, we want the probability that he was guessing. The answer is 371, so the correct option is (C).
Concept and intuition
This is a classic Bayes’ theorem problem. We have two ways a student can get a correct answer: either he knows the answer (and thus answers correctly for sure), or he guesses (and has a 1 in 4 chance of being correct). We are told the prior probability that he knows the answer is 90%. The question asks: given that he got it right, what’s the chance he was actually guessing? Bayes’ theorem lets us “flip” the conditional probability, using the known likelihoods.
Step-by-step reasoning
- Define events clearly Let K = “student knows the answer”, and G = “student guesses” (so G is the complement of K). Let C = “student gives the correct answer”. We are given:
P(K)=0.9,P(G)=0.1.
-
Conditional probabilities for answering correctly
- If the student knows the answer, he is certain to be correct: P(C∣K)=1.
- If he guesses, he picks randomly among 4 options, so P(C∣G)=41.
-
Find the total probability of a correct answer
By the law of total probability:
P(C)=P(C∣K)P(K)+P(C∣G)P(G)=(1)(0.9)+(41)(0.1)=0.9+0.025=0.925.
- Apply Bayes’ theorem We want P(G∣C), the probability that he was guessing given that he answered correctly:
- KCET 2022Set C-41 markMCQQ.If A and B are two events such that P(A)=21, P(B)=31 and P(A∣B)=41, then P(A′∩B′) is (A) 3/16 (B) 1/12 (C) 3/4 (D) 1/4
›Reveal solutionSolution
Get P(A∩B) from the conditional probability, use the addition rule for P(A∪B), then apply De Morgan: P(A′∩B′)=1−P(A∪B).
Step 1 — Recover the joint probability from the conditional.
Conditional probability is defined by P(A∣B)=P(B)P(A∩B) — it re-scales the probability of A to the reduced sample space B. Rearranging gives the multiplication rule:
P(A∩B)=P(A∣B)P(B)=41×31=121.
Step 2 — Addition rule for the union.
P(A∪B)=P(A)+P(B)−P(A∩B)=21+31−121.
Taking LCM 12: …
- COMEDK 2025Set 2025-A1 markMCQQ.Three bags contain a number of red and white balls are as follows. Bag I: 3 red balls Bag II: 2 red balls and 1 white ball Bag III: 3 White balls The probability that bag i will be chosen and a ball is selected from it is 6i,i=1,2,3. If a white ball is selected, what is the probablity that it came from Bag III (A) 119 (B) 112 (C) 0 (D) 111
›Reveal solutionSolution
This is a classic Bayes’ theorem problem: we are given prior probabilities for choosing each bag and the conditional probabilities of drawing a white ball from each bag; after observing a white ball, we want the posterior probability that it came from Bag III. The answer is 119.
We start by noting that the problem gives us three bags with different compositions, and a rule for choosing a bag: the probability of picking bag i is 6i. Then, from that bag, we pick a ball uniformly at random. We are told that a white ball was selected, and we need the probability that it came from Bag III. This is a textbook case for Bayes’ theorem, which lets us “reverse” conditional probabilities: we know P(white∣bag i), and we want P(bag i∣white).
Let’s work through it step by step.
- Define events and write down given probabilities. Let Bi be the event that bag i is chosen (i=1,2,3). We are told:
P(B1)=61,P(B2)=62,P(B3)=63.
Let W be the event that a white ball is drawn.
From the bag compositions:
- Bag I: 3 red, 0 white → P(W∣B1)=0.
- Bag II: 2 red, 1 white → P(W∣B2)=31.
- Bag III: 0 red, 3 white → P(W∣B3)=1.
- Compute the total probability of drawing a white ball. By the law of total probability:
P(W)=P(B1)P(W∣B1)+P(B2)P(W∣B2)+P(B3)P(W∣B3).
Substitute:
P(W)=61⋅0+62⋅31+63⋅1=0+182+63=91+21.
Get a common denominator (18):
P(W)=182+189=1811.
- Apply Bayes’ theorem to find P(B3∣W). Bayes’ theorem says:
P(B3∣W)=P(W)P(B3)P(W∣B3).
Plug in the numbers:
- KCET 2025Set A-11 markMCQQ.Meera visits only one of the two temples A and B in her locality. Probability that she visits temple A is 52. If she visits temple A, 31 is the probability that she meets her friend, whereas it is 72 if she visits temple B. Meera met her friend at one of the two temples. The probability that she met her at temple B is (A) 167 (B) 165 (C) 163 (D) 169
›Reveal solutionSolution
The friend has already been met (the effect); we want the probability of the cause (temple B) — that reversal of conditioning is exactly Bayes' theorem.
Step 1 — Name the events.
Let A = "Meera visits temple A", B = "Meera visits temple B", F = "she meets her friend".
She visits only one of the two temples, so A and B are mutually exclusive and exhaustive:
P(A)=52⟹P(B)=1−52=53.
Given: P(F∣A)=31, P(F∣B)=72.
Step 2 — Why Bayes.
We are told the outcome (F happened) and asked for the probability of a cause (B). Bayes' theorem inverts the conditioning:
P(B∣F)=P(A)P(F∣A)+P(B)P(F∣B)P(B)P(F∣B).
The denominator is P(F) by the law of total probability — the friend can be met on either branch.
Step 3 — Compute the two branch probabilities.
P(A∩F)=52×31=152,P(B∩F)=53×72=356.
Step 4 — Total probability of meeting the friend. …
- KCET 2020Set A-11 markMCQQ.If A and B are two events such that P(A)=31, P(B)=21 and P(A∩B)=61, then P(A′/B) is (A) 32 (B) 31 (C) 21 (D) 121
›Reveal solutionSolution
Use P(A′∣B)=1−P(A∣B) (equivalently P(B)P(B)−P(A∩B)), which gives 32.
Step 1 — The definition of conditional probability.
For P(B)>0,
P(A′∣B)=P(B)P(A′∩B)
Conditioning on B means we restrict the sample space to B and ask what fraction of B lies outside A.
Step 2 — Split B into the part inside A and the part outside A.
The events A∩B and A′∩B are disjoint and their union is exactly B:
P(B)=P(A∩B)+P(A′∩B)
⇒P(A′∩B)=P(B)−P(A∩B)=21−61=63−1=62=31
Step 3 — Divide by P(B).
P(A′∣B)=P(B)P(A′∩B)=2131=31×12=32
Step 4 — Cross-check with the complement rule. …
- COMEDK 2025Set 2025-E1 markMCQQ.If A and B are two events such that P(Aˉ)=0.3,P(B)=0.4,P(A∩Bˉ)=0.5, then find the value of P(B/A∪Bˉ) (A) 0.33 (B) 0.7 (C) 0.8 (D) 0.25
›Reveal solutionSolution
The problem asks for a conditional probability P(B∣A∪Bˉ). Using the definition of conditional probability and the given probabilities, we compute P(B∩(A∪Bˉ)) and P(A∪Bˉ), then divide. The final value is 0.25, so the correct option is (D).
We are given:
- P(Aˉ)=0.3, so P(A)=1−0.3=0.7.
- P(B)=0.4.
- P(A∩Bˉ)=0.5.
We need P(B∣A∪Bˉ), which is the probability that B occurs given that A or Bˉ (or both) occurs.
Concept and intuition:
Conditional probability P(X∣Y)=P(Y)P(X∩Y). Here X=B and Y=A∪Bˉ. So we need the probability that both B and (A∪Bˉ) happen, divided by the probability that A∪Bˉ happens. The key is to simplify the intersection B∩(A∪Bˉ) using set algebra — it often collapses to something simpler.
-
Simplify the numerator P(B∩(A∪Bˉ))
Using the distributive law:
B∩(A∪Bˉ)=(B∩A)∪(B∩Bˉ)
But B∩Bˉ=∅, so this is just B∩A.
Hence P(B∩(A∪Bˉ))=P(A∩B).
-
Find P(A∩B)
We know P(A)=0.7 and P(A∩Bˉ)=0.5.
Since A=(A∩B)∪(A∩Bˉ) and these are disjoint,
P(A)=P(A∩B)+P(A∩Bˉ)
⇒0.7=P(A∩B)+0.5
⇒P(A∩B)=0.2.
-
Find the denominator P(A∪Bˉ)
Use the inclusion-exclusion principle: …
- COMEDK 2026Set 2026-A1 markMCQQ.Vishnu has two jars of marbles, Jar A and Jar B. Jar A contains 3 yellow marbles and 2 green marbles. Jar B contains 4 yellow marbles and 3 green marbles. Vishnu flips a fair coin. If it lands heads, he picks two marbles at random without replacement from Jar A. If it lands tails, he picks two marbles at random with replacement from Jar B. Given that Vishnu picked one yellow and one green marble, what is the probability that they came from Jar B? (A) 4121 (B) 8949 (C) 8940 (D) 4120
›Reveal solutionSolution
P(E∣A)=53 (without replacement), P(E∣B)=4924 (with replacement); Bayes gives P(B∣E)=8940 — option (C).
Likelihoods of drawing one yellow and one green (E).
Jar A (3 yellow, 2 green; two draws without replacement):
P(E∣A)=(25)(13)(12)=106=53.
Jar B (4 yellow, 3 green; two draws with replacement):
P(E∣B)=2⋅74⋅73=4924.
Bayes' theorem with P(A)=P(B)=21 (the 21 cancels): …
- COMEDK 2026Set 2026-M1 markMCQQ.Advika chooses one of three scarves every morning: Red, Blue, or Green. The probability she chooses Red is 20%. The probability she chooses Blue is twice the probability of choosing Red. On the remaining days she wears a Green scarf. Once a scarf is chosen, she decides whether to wear a Hat (H) and Sunglasses (S). These choices are independent of each other but depend on the scarf colour: Scarf colour Red Blue Green P(H)0.50.40.1P(S)0.80.50.5 Advika is spotted outdoors wearing both a Hat and Sunglasses. What is the probability that she is wearing the Red scarf? (A) 31313 (B) 218 (C) 94 (D) 138
›Reveal solutionSolution
Bayes' theorem on scarf colour given that both a hat and sunglasses are worn. Priors P(R)=0.2, P(B)=0.4, P(G)=0.4; likelihoods P(H∩S∣colour)=P(H)P(S). The posterior P(R∣H∩S)=94 — option (C).
Concept. Hat and sunglasses are independent given the scarf, so P(H∩S∣colour)=P(H∣colour)⋅P(S∣colour). Bayes' theorem then reverses the conditioning to give the probability of the scarf colour from the observed accessories.
Step 1 — Priors.
P(R)=20%=0.2,P(B)=2P(R)=0.4,P(G)=1−0.2−0.4=0.4.
Step 2 — Likelihood of wearing both accessories for each colour.
P(H∩S∣R)=0.5×0.8=0.40,
P(H∩S∣B)=0.4×0.5=0.20,
P(H∩S∣G)=0.1×0.5=0.05. …
- KCET 2020Set A-11 markMCQQ.The probability of solving a problem by three persons A, B and C independently is 21, 41 and 31 respectively. Then the probability of the problem is solved by any two of them is (A) 121 (B) 41 (C) 241 (D) 81
›Reveal solutionSolution
The problem asks for the probability that exactly two of the three persons solve it. We compute the sum of probabilities for each pair solving while the third fails, giving 81.
The key idea: "solved by any two of them" means exactly two solve it, not at least two. The third person must fail. Since A, B, and C work independently, we multiply their individual probabilities for each specific outcome and then add the three possible cases.
A common mistake is to include the case where all three solve it. That would be "at least two", not "any two". The phrase "any two" in probability problems almost always means exactly two.
Let’s denote:
- P(A)=21, so P(A fails)=1−21=21
- P(B)=41, so P(B fails)=1−41=43
- P(C)=31, so P(C fails)=1−31=32
We want exactly two successes. There are three mutually exclusive ways this happens:
-
A and B solve, C fails
Probability = P(A)×P(B)×P(C fails)
=21×41×32=242=121
-
A and C solve, B fails
Probability = P(A)×P(C)×P(B fails)
=21×31×43=243=81
-
B and C solve, A fails
Probability = P(B)×P(C)×P(A fails) …
- COMEDK 2026Set 2026-M1 markMCQQ.Samhita faces a three-headed dragon. She wins a "Tactical medal" if she manages to defeat exactly one of the three heads. The battle proceeds head-by-head under the following conditions: The probability of defeating the first head is 31. After a win: if she defeats a head, the probability of defeating the next head is 32. After a loss: if she fails to defeat a head, the probability of defeating the next head is 41. What is the probability that Samhita earns the "Tactical medal"? (A) 7223 (B) 365 (C) 7217 (D) 7219
›Reveal solutionSolution
Summing the three disjoint "exactly one win" paths gives 121+181+81=7219 — option (D).
Set up the conditional probabilities. Let Wi mean "defeats head i" and Li mean "fails":
- P(W1)=31, so P(L1)=32.
- After a win: next-head win probability =32, so next-head loss probability =31.
- After a loss: next-head win probability =41, so next-head loss probability =43.
Earning the medal means exactly one of the three heads is defeated. The three disjoint sequences are W1L2L3, L1W2L3, and L1L2W3.
Path 1 — W1L2L3 (win, then loss after a win, then loss after a loss):
P=31⋅31⋅43=363=121
Path 2 — L1W2L3 (loss, then win after a loss, then loss after a win): …
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