a wheel spins at a rate of 9 revolutions for 5 seconds. How long will it take to spin for 20 revolutions?
5 / 9 = x / 20 cross-multiply
20 * 5 = 9 * x divide both sides by 9
[ 20 * 5 ] / 9 = x
100 / 9 = x = 11 + 1/9 seconds
+4 
