Question

calculate the focal length of a convex lens which produces a virtual image at a distance of 50 centimetre of an object placed 20 centimetre in front of it

Answer 1

given ,
u=-20 cm
v=50 cm
f=?
use lens formula ,
1/f=1/v -1/u

1/f=1/50 +1/20

1/f=7/100

f=+100/7cm

Answer 2

Object distance (u) = -20cm
Image distance (v) = -50cm
Focal length (f) =?

1/v - 1/u = 1/f
1/f= 1/-50 +1/20
=-2/100+5/100
=3/100

1/f= 3/100
So f=100/3
=33.3