According to the given data we have the following:
family orders 3 large pizzas
The teenager in the family eats 1/4 of the family's order
Therefore, in order to calculate the amount of pizza he eat we would make the following calculation:
amount of pizza he eat=quantity of pizza the teenage eat*total pizzas
amount of pizza he eat=1/4*3
amount of pizza he eat=3/4
Therefore he will eat 3/4 of a large pizza.