Answer
$0$
Work Step by Step
From the given graph, apply the formula such as: $\dfrac{P(x_2)-P(x_1)}{x_2-x_1}$
Plug the given data in the above formula.
we have
$\dfrac{P(x_2)-P(x_1)}{x_2-x_1}=\dfrac{P(40)-P(20)}{40-20}$
This gives:
$\dfrac{P(40)-P(20)}{40-20}=\dfrac{20-20}{20}$
or, $\dfrac{P(40)-P(20)}{40-20}=0$
