Question

From 1940 to 1980, the consumer price index (CPI) increased from 28.6 to 89.4. If a gallon of milk cost $0.75 in 1940 and the price of milk increased at the same rate as the CPI from 1940 to 1980, approximately how much did a gallon of milk cost in 1980?

Answer 1

28.6 / 89.4 = 0.75 / x
cross multiply
28.6x = 67.05
x = 67.05 / 28.6
x = 2.34 <==