Answer
$i = 28~cm$
Work Step by Step
When $p = 20~cm$, we can see that $i$ is undefined.
We can find $r$:
$\frac{1}{p}+\frac{1}{i} = \frac{2}{r}$
$\frac{1}{i} = \frac{2}{r}-\frac{1}{p} = 0$
$\frac{2}{r} = \frac{1}{p}$
$\frac{2}{r} = \frac{1}{20~cm}$
$r = 40~cm$
We can find $i$ when $p = 70~cm$:
$\frac{1}{p}+\frac{1}{i} = \frac{2}{r}$
$\frac{1}{i} = \frac{2}{r}-\frac{1}{p}$
$\frac{1}{i} = \frac{2}{40~cm}-\frac{1}{70~cm}$
$\frac{1}{i} = \frac{1}{28~cm}$
$i = 28~cm$