The pta is ordering pizza for their next meeting. they plan to order 2 cheese pizzas for every 3 pepperoni pizzas they order. how many cheese pizzas will they order if they order 15 pepperoni pixzas?

10 cheese pizzas

use  simple ratio, 3:2 (3 pepperoni for every 2 cheese)
to make the ratio bigger, we substitute the "3" pepperoni for "15" pepperoni. then take the new amount, 15/3 original, you get 5. then multiply the 2 original cheese pizzas by that 5, and you get 10 cheese pizza's. 

hope that helps