A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens ?

Given f = -15 cm. v= -10 cm, u = ?

Lens formula 1/f = 1/v- 1/u

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

1/u = -1/10 + 1/15 = -3 +2 /30 = -1/30

u = -30 cm

So the object should be placed 30 cm from the lens.