There is 3/4 of a box of cereal remaining. You eat 2/5 of the remaining cereal. What fraction of the box did you eat?

First, you find a common denominator, 20, then 3•5, (because you need to times 4 by 5 to get 20) which is 15, so the first fraction is 15/20. The. The same thing with 2/5, 2•4 which is 8 so your second decimal is 8/20. finally, you subtract, 15-8=7 so you ate 7/20 of the box