Answer
$f = 67.8~cm$
Work Step by Step
We can find the focal length:
$\frac{1}{f} = (n-1)(\frac{1}{R_1}-\frac{1}{R_2})$
$\frac{1}{f} = (1.59-1)(\frac{1}{40~cm}-\frac{1}{\infty})$
$\frac{1}{f} = (0.59)(\frac{1}{40~cm})$
$f = \frac{40~cm}{0.59}$
$f = 67.8~cm$
