If you used 3/4 of the blue paint to make flames on one of the cars, and need 7 3/4 pints of blue paint to paint another car, what amount of blue paint do you need to buy?

3/4 of 7 pints were used. of, in this case, means to multiply. 3/4 x 7 = 21/4 = 5 and 1/4 pints. Now we subtract 5 and 1/4 (because this is how much was used) from 7. 7 - 5 and 1/2 = 1 and 1/2

If you have questions, feel free to ask.