Please Help! I don't know how to solve this question and I need to learn by today; I have an exam tomorrow :(

Question

A Pizzeria sells pizza according to size: small pizzas cost $10, medium pizzas cost $15, and large pizzas cost $40. They usually sell as many small pizzas as medium and large pizzas combined. The number of medium pizzas sold is usually twice as many as large ones. How many of each size pizza must they sell to get $600?

Answer 1

10S + 15M + 40L = 600

S=M+L

M=2L

S=2L+L = 3L

10(3L) +15(2L)+40L = 600

30L +30L +40L =600

100L =600

L= 600/100 = 6 large sold

M=2L = 12 medium sold

S=M+L = 18 small sold

18(10)+12(15)+40(6) = 180+180+ 240 =$600 total revenue