Answer :
If we want to use red to yellow in ratio 3:5 and we know we have to use 20 oz of yellow we can say that our 5 from the ratio represents 20 oz. So we can divide it by 5 to find how much oz is 1 and then multiply it by 3 to find amount of red.
20:5=4 [oz]
4*3=12 [oz] - amount of red
20:5=4 [oz]
4*3=12 [oz] - amount of red
So on a paper you write: 3:5 since the yellow is 5 u put. 20 under the 5 now you can cross multiply(20•3=60) and now divide 60 by 5 which is 12