Answer
$i = -18~cm$
Work Step by Step
We can find $i$:
$\frac{n_1}{p}+\frac{n_2}{i} = \frac{n_2-n_1}{r}$
$\frac{n_2}{i} = \frac{n_2-n_1}{r} - \frac{n_1}{p}$
$\frac{1.5}{i} = \frac{1.5-1.0}{30~cm} - \frac{1.0}{10~cm}$
$\frac{1.5}{i} = \frac{0.5}{30~cm} - \frac{3.0}{30~cm}$
$\frac{1.5}{i} = -\frac{2.5}{30~cm}$
$\frac{i}{1.5} = -\frac{30~cm}{2.5}$
$i = -18~cm$