Question

A concave lens has focal length of 15cm.At what distance should the object from the lens be placed so that if forms an image 10cm from the lens?

Answer 1

lens is concave therefore,
focal length = -15 cm
image distance, V = -10 cm
we know that , 1/f=1/V-1/u
therefore , answer is u= -30 cm.
hope I am right

Answer 2

f= -15cm. v= -10cm

1/v-1/u=1/f

1/-10-1/u=1/-15

1/-10+1/15=1/u

-3+2/30=1/u

-1/30=1/u

u= -30