Application of calculus

A lamp is placed on the ground 100ft away from a wall. a max 6ft tall is walking at a speed of 10ft/sec from the lamp to the nearest point on the wall. When he is midway between the lamp and the wall , the rate of change in the length of his shadow is (in ft/sec)?

a. 2.4
b. 3
c. 12
d. 3.6

Thanks

On my website you'll find similar problems with animations

Got to Calculus Animations

On the optimization and related rates page see walking towards the light and walking away from the light