To get the purple I need I mix 1/3 cups of blue and 1/2 cups of blue. How many cups of blue and red do I need to get 20 cups of purple

20

Step-by-step explanation:

If you combine 1/3 c of blue & 1/2 c of red, you get 5/6 c of purple.

(5/6) x = 20

x = (6/5) 20 = 24; this multiplier is applied to each of the above measurements to show that you need 8 c of blue + 12 c of red.