The coverging lens used for magnification is called simple microscope or a magnifier.

When image is formed at $D$, the least distance of distinct vision, then magnifying power

$M=1+\frac{D}{f}$

Given, $D$= 25 cm, $f$ = 10 cm

$\therefore M=1+\frac{25}{10}=1+2.5=3.5$