An object is placed 60 cm in front of a convex lens with a focal length of 20 cm. where does the image form?

a. 15 cm
b. 30 cm
c. -15 cm
d. -30 cm

We are given with the focal length f equal to 20 cm and distance of the object do equal to 60 centimeters. We can compute for the distance of the image di from the lens equation
     1/f = 1/do + 1/di
     1/di = 1/f - 1/do = (do - f) / do f
     di = do f / (do - f)
Substituting the values to the derived equation, we can now calculate for the image distance di:
     di = do f / (do - f)
     di = 60*20 / (60 - 20) = 30 cm
Therefore, the convex lens will produce an image at 30 centimeters.