Question

at what distance from a concave lens of focal length 20cm be placed so as to obtain it's image at 15cm from the lens​

Answer 1

Answer is 60cm

Explanation:

Given:

f = -20 cm

v = -15 cm

Formula:

1/f = 1/v - 1/u

1/-20 = 1/-15 - 1/u

1/u = 1/15-1/20

1/u = 4-3/60

1/u = 1/60

u = 60cm

hope it helps you....