hihi! so the ratio 2:1:3 can be rewritten as 2/6 (red) 1/6 (blue) and 3/6 (yellow).
this is because you have 2 floz of red, 1 floz of blue, and 3 floz of yellow, which adds up to 6 floz of paint total in your ratio.
since you have 18 fl. oz. of paint, you multiply the ratio 3/6 by 18 to get 9 fl. oz. of yellow paint.
hope this helps!
