Hello! So, we could set up a proportion to solve this, because the price and CPI increased at the same rate. Let's set it up like this: 34.6/82.4 = 0.75/x. This is because we know the prices from the 1960's, but we don't know the gallon of milk price in 1980, and we line them up like that to make sure we get the proper answer. Cross multiply the values in order to get 34.6x = 61.8. Now, divide each side by 34.6 to isolate the variable. x ≈ 1.7861 and other numbers behind it or 1.79 when rounded to the nearest hundredth. The price for a gallon of milk in 1980 was about $1.79.
