Answer:
4 pizzas are needed.
Explanation:
See image. You could totally think this through and use mental math to figure out how many pizzas are needed. See first image. Picture a pizza, cut it into 6 slices. It feeds 6 people. 2 pizzas feed 12 people. 3 pizzas feed 18 people. 4 pizzas feed 24 people.
If you have to write equations and show math work, you can write a single-variable equation or use a proportion. See image.