The least distance of distinct vision of an average person is 25 cm in order to view an object with magnification of 10.
Here v = D = 25 cm and u = f
But the magnification, m=v/u = D/f
m = D/F
====>f=D/m=25/10 = 2.5=0.025m
P = 1/0.025 = 40 D
This is the required power of lens.