The focal length of a thin biconvex lens is 20cm. When an object is moved from a distance of 25cm in front of it to 50cm, the magnification of its image changes from m25 to m50. The ratio m25/m50 is :

When object is at 25cm u = 25cm,
Using lens formula 1/v - 1/u = 1/f
1/v - 1/25 = 1/20
1/v = 1/20 + 1/25 = (5+4)/100 = 9/100
v = 100/9 cm
m25 = v/u = (100/9)/25 = 4/9 (1)
When object is at 50cm u = 50cm
1/v - 1/50 = 1/20
1/v = 1/20 + 1/50 = (5+2)/100 = 7/100
v = 100/7 cm
m50 = v/u = (100/7)/50 = 2/7 (2)
We have to find m25/m50
m25/m50 = (4/9)/(2/7) = (4/9)*(7/2) = 14/9