Answer
$f = 30~cm$
Work Step by Step
We can find the focal length:
$\frac{1}{f} = (n-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f} = (1.50-1)(\frac{1}{24~cm}-\frac{1}{-40~cm})$
$\frac{1}{f} = (0.50)(\frac{5}{120~cm}+\frac{3}{120~cm})$
$\frac{1}{f} = (0.50)(\frac{1}{15~cm})$
$\frac{1}{f} = \frac{1}{30~cm}$
$f = 30~cm$
