How many gallons of 45% butterfat cream must be mixed with 500 gallons of 3% butterfat milk to obtain a 5% butterfat milk?

Answer 1

fat = f

.45 f + .03 (500) = .05 (500+f)
or
.40 f = .02(500)

Answer 2

Let x = gallons 45%
45x + (500*3) = (500+x)*5