Cody has a box of cereal. on monday he ate 1/7 of the cereal. on tuesday, he ate 1/6 of the remaining cereal. what fraction of the box is left in the box

So, on Monday he ate 1/7, that means from a whole 7/7, take away 1/7, the leftover was 6/7 then.

on Tuesday, he ate 1/6 of the leftover, namely, 1/6 of 6/7.

now, how much is 1/6 of 6/7?  well, is just their product,   

now, so he really ate 1/7 of the whole thing on Tuesday, there were 6/7 remaining, how much is 6/7 - 1/7?  yeap, you guessed it, 5/7.