There is 3/4 of a box of cereal remaining. you eat 2/5 of the remaining cereal. what fraction of the box do you eat?

Change the problem so there is a common denominator
15/20 ; 8/20
you eat 8/20 of the 15/20 cereal
therefore, multiply the two numbers 
6/20 is the remaining amount of cereal in the box
3/4 or 15/20 of the box was there to start
but the questions wants how much of the ENTIRE box was eaten
with 6/20 of the entire box remaining and 15/20 of the box was left before you ate some of the cereal
15/20-6/20= 9/20 of the box
you ate 9/20 of the entire box