Marty didn't eat his crusts ...
or, Rob had a larger pizza.
Marty ate 1/4 of a pizza. Rob ate 1/5 of a pizza. How could Rob have eaten more pizza than Marty?
2 answers
If you put one whole and put it next to 1/5 then it would be bigger,so it would be 1 whole and 1/5 is greater than 1/4.