Harry eats 2/3 of his pizza. Charlotte eats 7/9 of her pizza.

a) what fraction of pizza has been eaten in total? - I got 13/9 (1 and 4/9)

b) what total fraction of pizza is left?

The first part of your question, you got correct ... 13/9 or 1 4/9.

We can assume 2 pizzas because they each ate a part of their pizza. So to get how much is left we just subtract your answer from 2.

2 - 1 4/9 = 5/9

5/9 is left over.