Q.The demand function for a commodity is $Q_d = 120 - 3P$, where $Q_d$ is the quantity demanded and $P$ is the price in ₹.

The demand function is of the form Qd = a + bP, which is a linear function; substitute the given values directly. …

(i) Nature of the function.

$Q_d = 120 - 3P$ is of the form $Q_d = a + bP$ with $a=120$, $b=-3$ — a linear function of $P$. The slope (rate of change of $Q_d$ with respect to $P$) is $-3$: for every ₹1 rise in price, quantity demanded falls by 3 units, consistent with the Law of Demand. The intercept (value of $Q_d$ when $P=0$) is $120$.

(ii) Quantity demanded at $P=20$.

(iii) Price at which $Q_d = 60$.

$60 = 120 - 3P \Rightarrow 3P = 120-60 = 60 \Rightarrow P = ₹20$ …

Confusing the slope's sign — writing slope = 3 instead of −3 (the coefficient of P, including its sign, IS the slope). Also, solving part (iii) as if Qd = 60 were a new independen …