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

Given: f = -15 cm, v = -10 cm
From the lens formula, 1/v - 1/u = 1/f
1/(-10) - 1/u = 1/(-15)
-1/10 - 1/u = -1/15
-1/u = -1/15 + 1/10
-1/u = (-2 + 3)/30
-1/u = 1/30
u = -30 cm
Therefore, the object is placed 30 cm from the lens.