Hey there!
You have to find a common denominator. The common denominator is 60. To get 1/5 to a denominator of 60, multiply the top and bottom by 12. You get 12/60. To get 9/12 to a denominator of 60, multiply the top and bottom by 5. You get 45/60. 45/60 is more than 12/60, so the pizza that was 9/12 eaten was more. They ate more of the pepperoni pizza.
I hope this helps!