Answer
$26cm$
Work Step by Step
We can find the required focal length of the lens as follows:
$\frac{1}{f}=\frac{1}{d_{\circ}}+\frac{1}{d_i}$
We plug in the known values to obtain:
$\frac{1}{f}=\frac{1}{\infty}+\frac{1}{d_i}$
We plug in the known values to obtain:
$\frac{1}{f}=\frac{1}{infty}+\frac{1}{26cm}$
$\frac{1}{f}=\frac{1}{26cm}$
$\implies f=26cm$
