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Worked Examples · Example 8

Q.AA and BB are independent events with P(A)=0.3P(A)=0.3 and P(B)=0.6P(B)=0.6. Find

(i) P(A∩B)P(A\cap B),
(ii) P(A∪B)P(A\cup B),
(iii) P(A∣B)P(A\mid B).
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(i) Because AA and BB are independent,

P(A∩B)=P(A)⋅P(B)=0.3×0.6=0.18.P(A\cap B)=P(A)\cdot P(B)=0.3\times 0.6=0.18.

(ii) By the addition theorem,

P(A∪B)=P(A)+P(B)−P(A∩B)=0.3+0.6−0.18=0.72.P(A\cup B)=P(A)+P(B)-P(A\cap B)=0.3+0.6-0.18=0.72.

(iii) By definition P(A∣B)=P(A∩B)P(B)=0.180.6=0.3P(A\mid B)=\dfrac{P(A\cap B)}{P(B)}=\dfrac{0.18}{0.6}=0.3. This equals P(A)P(A), which is exactly what independence means — knowing BB occurred does not change the chance of AA. …

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