Explanation:
1 cup of yellow paint = 2×1/2 cup.
and that requires then also 2×1/3 Co red paint.
so, 1 cup of yellow paint mixes with 2/3 cups of red paint.
2 and 3 have a smallest common multiple (smallest number that is divisible by both numbers without remainder) of 6.
so, we need to get 6 units of each for a whole number ratio :
6×1/2 = 3 cups of yellow paint mix with 6×1/3 = 2 cups of red paint.