A pencil (AB) of length 20 cm is moving along the principal axis of a concave mirror MM', with a velocity 5 m/s approaching the mirror. The mirror itself is moving away from the pencil at a speed of 2 m/s. Find the rate of change of length of the image of the pencil at the instant end A is at a distance of 60 cm from the mirror. Focal length of the mirror is 20 cm.