The sprocket is the big gear in the middle.

So: 42/ 14=3.

In other words, every time a person walks, the wheel behind him is three times.

Also: the diameter of the bicycle wheel is 60 cm, so the circumference of the wheel is pai*60=60pai cm.

The distance traveled in one minute is12km/h =12 *1000/60min = 200m.

200* 100 cm /60 rows = 1000/3 rows of circles

The revolution of the sprocket in one minute is: (1000/3pai)/3 =1000/9pai, which is about 35.39 revolutions (so the answer should be an integer of 36 revolutions).