Answer
$p = 5.0$
Work Step by Step
We can find $p$:
$\frac{1}{p}+\frac{1}{i} = \frac{2}{r}$
$\frac{1}{p} = \frac{2}{r}-\frac{1}{i}$
$\frac{1}{p} = \frac{2}{40}-\frac{1}{-4.0}$
$\frac{1}{p} = \frac{4}{20}$
$p = 5.0$
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