A man 1.8 m tall walks at speed of 10m/s towards a street lamp which is 7m above the ground. How fast is the length of the man's shadow decreasing? My attempt: Let the man's shadow and the lamp's length be l, and the distance be d and time be t. Given, dd/dt = 10. I am supposed to find dl/dt. dd/dt= dl/dt X dd/dl So how do I find dd/dl?