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A lens of focal length 15cm forms an erect image which is...

A lens of focal length 15cm forms an erect image which is three times the size of the object. The distance between the object and the image is ___

A.

10cm

B.

20cm

C.

30cm

D.

40cm

Correct answer is D

F= 15cm

  M = 3

  M = vu

  V =3u

  1f = 1u + 1v

  115 = 1u - 13u

  115 =   23u

U =10cm

  D = u + v

  = 10 +30 = 40cm