calculus related rates

Question

A man 6-ft tall walks at a rate of 5 ft/sec. away from a lamppost that is 22-ft high. At what rate is

the length of his shadow changing when he is 60 feet away from the lamppost?

Answer 1

Let y is his distance from lamp and x is length of shadow. Then from similarity of triangles:; 8x = 22/6 = (x + y)/x; 11x = 3x + 3y; x = 3/8y; dx/dt = 3/8dy/dt; dx/dt = 3/8·5 ft/s = 1.875 m/s and independent of y