if a pound of grapes cost $0.14 in 1960 and the price of grapes increased at the same rate as the CPI from 1960 to 1980 by approximately how much did the price of a pound of grapes increase

The CPI increased from 29.6 to 82.4.

The rate of increase, is:

82.4/29.6 = 2.78

Now multiply this rate to the $0.14 price of grapes from 1960.

$0.14 x 2.78 = $0.39

Now to get the amount of increase subtract the new price from the original price:

$0.39 - $0.14 = $0.25 increase.