Answer: Mark ate
of the entire pizza on Friday.
Step-by-step explanation: Given that Mark left
of a pizza in the refrigerator and on Friday, he ate
of what was left on the pizza.
We are to find the fraction of the entire pizza that Mark ate on Friday.
Since Mark ate half of the pizza that was left in the refrigerator, so the fraction of the entire pizza that Lark ate on Friday is

Thus, Mark ate
of the entire pizza on Friday.