Brine is a solution of salt and water. if a tub contains 50 gallons of a 5% solution of brine, how much water must evaporate to change it to an 8% solution?

Let x = amt of water evaporated

.05(50) = .08(50-x)
2.5 = 4 - .08x
.08x = 4 - 2.5
.08x = 1.5
x = 
x = 18.75 lb of water evaporated

Check; amt of salt remains the same, only the percentage is different, right?
.05(50) = .08(50-18.75)
.05(50) = .08(31.25)
2.5 = 2.5