At pauls pizza shop, 3 pepperonipl pizzas are sold for every 2 cheese pizzas sold. if 12 cheese pizzas are sold during a 20 minute period, how many pepperoni pizzas would the restaurants expect to sell during this time?

Given:
3 pepperoni pizza for every 2 cheese pizzas sold
12 cheese pizzas sold.

This is a proportion problem:

3 : 2 = x : 12

a : b = c : d  where ad = bc

a = 3 ; b = 2 ; c = x ; d = 12

3 * 12 = 2 * x
36 = 2x
36/2 = 2x/2
18 = x

18 pepperoni pizzas are sold during the 20 minute period.

Its four because 3x4=12 that's really all you have to do.