Yellow:blue is a ratio of 2:3, meaning that 2/5 of the desired mixture is yellow. From this, you need to find how much yellow is necessary for 20 ounces. Well, 2/5 of the 20 ounces will be yellow paint, while the other 3/5 will be blue. So she will need 8 ounces of yellow paint to get 20 ounces of the desired mixture. Hope this helps! :)
3 ounces of blue paint added with 2 ounces of yellow you would get 5 onces Then you divide 20 by 5 you get 4 Then multiply 4 by 2 ounces you get 8 Sheila will need 8 ounces of of yellow paint for the new mixture.