How many gallons of milk containing 5% butterfat must be mixed with 90 gallons of 1% milk to obtain 2% milk?

Answer 1

gallons of 5% milk --- x

.05x + .01(90) = .02(x+90)
times 100
5x + 1(90) = 2(x+90)
5x + 90 = 2x + 180
3x = 90
x = 30