Turn the 1/4 into a decimal which is 0.25 and times that by 20 and you’ll get 5, 5 is the amount of ounces you poured for your friend. Subtract 20 ounces by 5 ounces and you’ll get 15 ounces, then subtract 15 by 3 which is the amount of juice you poured for yourself and you’ll be left with 12 ounces in the bottle.
