Answer :
If john ate 1/2 on Friday, then there was a full half of pizza left when he finished.
And if he ate 1/3 of what was left on Saturday, there was only 2/3 of the pizza left for him to eat. So he ate 2/3 of a pizza on Sunday
And if he ate 1/3 of what was left on Saturday, there was only 2/3 of the pizza left for him to eat. So he ate 2/3 of a pizza on Sunday