Answer
$+10cm$
Work Step by Step
First we consider equation (34-8):
$\frac{n_1}{p}+\frac{n_2}{i}=\frac{n_2-n_1}{r}$
Solving for the object distance, $p$, we obtain:
$p=n_1(\frac{n_2-n_1}{r}-\frac{n_2}{i})^{-1}$
Using the given values in the table, we have:
$p=1.5(\frac{1.0-1.5}{-30cm}-\frac{1.0}{-7.5cm})^{-1}$
$p=+10cm$