Linda needs 20 gallons of juice that has 75%fruit juice. she has a mixture that has 80% fruit juice and another mixtures that has 60% fruit juice. how much of each mixture should she use to get what she need?

Linda should use ...
  15 gallons of 80% juice
  5 gallons of 60% juice


Let g represent the number of gallons of 80% juice needed. Then (20-g) is the number of gallons of 60% juice Linda will use. The amount of juice in the mixture can be written as
  0.80g +0.60(20 -g) = 0.75×20
  0.20g = 20(0.75 -0.60) = 20×0.15
  g = 20×0.15/0.20 = 15
The amount of 80% juice needed is 15 gallons, so 5 gallons of 60% juice are needed.