Answer
$D(40)$ = $22.5$ $barrels/year$
increase = $-0.5625$
decrease = $+0.5625$
Work Step by Step
$D(40)$ = $\frac{900}{40}$ = $22.5$ $barrels/year$
increase
$D'(p)$ = $-\frac{900}{p^{2}}$
$D'(40)$ = $-\frac{900}{40^{2}}$ = $-0.5625$
decrease
$-D'(40)$ = $\frac{900}{40^{2}}$ = $+0.5625$
