Magnification of a compound microscope is 30. Focal . . .

Question : Magnification of a compound microscope is 30. Focal length of eye piece is 5 cm and the image is formed at a distance of distinct vision of 25 cm. The magnification of the objective lens is :

a) 6

b) 5

c) 7.5

d) 10

Doubt by Angad

Solution : 

m=-30 
f_e=5 cm

We know, 

m_e=(1+D/f_e)

m_e=(1+25/5)
m_e=1+5

m_e=6

Also 

m=m_o×m_e

-30=m_o×(6)
-30/6=m_o
m_o=-5
m_o=|-5|
m_o=5

Hence, b) 5 would be the correct option.