Final answer:
Jacob bought 3 medium and 6 large pizzas for his class party, as the large pizzas were twice the amount of medium pizzas and the total number was 9.
Step-by-step explanation:
Jacob bought a total of 9 pizzas for his class party. The condition given is that the amount of large pizzas was twice the amount of medium pizzas. Let's define the number of medium pizzas as x. Therefore, the amount of large pizzas would be 2x. Since the total number of pizzas is 9, we can write the equation x + 2x = 9. Adding the terms together gives us 3x = 9. Dividing both sides of the equation by 3, we find that x = 3. This means Jacob bought 3 medium pizzas and 2 times 3, which is 6 large pizzas for his class party.