Answer:
An equation can be used to find x, the number of gallons that should be evaporated is:
13.5 / (90 - x) = 25/100
Explanation:
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