Question

Carlon plans to evaporate a number of gallons of water from 90 gallons of a 15% salt solution to make a 25% salt solution. Which equation can he use to find x, the number of gallons that should be evaporated?

Answer 1

The first thing we will do for this case is define variables.
 We have then:
 x = amount of water to evaporate.
 90-x = amount of water after being evaporated.
 The amount of salt is:
 S = 90 * 0.15 = 13.5
 The percentage of salt will be:
 % S = 13.5 / (90 - x) = 25/100
 Answer:
 An equation can be used to find x, the number of gallons that should be evaporated is:
 13.5 / (90 - x) = 25/100