Answer:
You use 2 cups of blue paint.
Explanation:
In this case, you can use a rule of three to find the amount of blue paint you need when you use 4 cups of yellow paint given that you mix 1/3 cups of blue paint for every 2/3 cup of yellow paint:
2/3 cup of yellow paint → 1/3 cups of blue paint
4 cups of yellow paint → x
x=(4*1/3)÷2/3
x=4/3÷2/3
x=4*3/3*2
x=12/6
x=2
According to this, the answer is that you use 2 cups of blue paint.