c + s = 132/11 (1) (the rate traveling with sidewalk) c - s = 60/15 (2) (the rate traveling against sidewalk) Or c + s = 11, (1') c - s = 4 (2') Add (1') and (2'): 2c = 15 ---> c == 7.5 ft per second is the rate of the child on a still sidewalk. ===> s = 11-7.5 = 3.5 ft per second is the rate of sidewalk.