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?

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