Answer
$12.5cm$
Work Step by Step
We can find the focal length of the magnifying glass as follows:
$M_{near\space point}=1.5M_{inf}$
$\implies 1+\frac{N}{f}=1.5(\frac{N}{f})$
$\implies 1=(1.5-1)\frac{N}{f}$
This simplifies to:
$f=(0.5)N$
$\implies f=(0.5)(25cm)=12.5cm$
