Answer:
So first let's start with Antonio's pizza, we have to pay $17.75 plus $1 per toppings.
Then, we have Papa's pizza, we have to pay $15, plus $2.25 per topping.
How will we find the number of toppings for the same price?
Well, at papa's pizza one topping already $17.25 dollars already. So, lets keep adding. Well, if we get 4 toppings at Antonio's pizza we have to pay $21.75. And if we 3 toppings at Papa's pizza we have to pay $21.75 as well.
So, at Antonio's pizza we have to get 4 toppings and at Papa's pizza we have to get 3 toppings, to be able to pay the same price at both places
SIMPLE EQUATION:
Papa's Pizza: $15 + $6.75 (3 toppings total) = $21.75
Antonio's Pizza: $17.75 + $4 (4 toppings total) = $21.75