Question

an object is 25 cm away from a convex len.the height of the image is twice the height of the object.find it's focal length.

Answer 1

Object distance = 25cm,  u = 25cm
 
Image = 2 = v/u;       v = 2*u = 2*25 = 50cm

By mirror formula:

1/u + 1/v = 1/f         

1/25 + 1/50 = 1/f

1/f = 1/25 + 50

1/f = (2 + 1)/50 

1/f = 3/50

f = 50/3 

f ≈ 16.667 cm