u = 40 cm (negative)
f = 40 cm (positive)
Mirror formula:
1\over u + 1\over v = 1\over f
⇒ – 1\over u + 1\over v = 1\over 40
⇒ 1\over v = 1\over 40 + 1\over u
⇒ 1\over v = 2\over 40
⇒ 1\over v = 1\over 20
⇒ v = 20 cm
Hence, the image is formed 20 cm behind the mirror.