A wheel has a diameter of x inches and a second wheel has a diameter of y inches. The first wheel covers a distance of d feet in 100 revolutions. How many revolutions does the second wheel make in covering d feet?
You'd want an expression which relates to x, y and d. The distance d is x times pi times 100. One divides back by pi and by y to get the # of revolutions (that 100 :)) ) and so one gets \(\frac{100x}{y}\) as the number of revolutions.