Answer:
[tex]22.5-0.25x=13.5[/tex]
36 gallons of water need to be evaporated.
Step-by-step explanation:
Let x gallons be the amount of water needed to be evaporated. Then (90 - x) gallons of solution will left.
1. Calculate the amount of salt in 90 gallons of a 15% salt solution:
[tex]90\cdot 0.15=13.5[/tex]
2. Calculate the amount of salt in (90 - x) gallons of a 25% salt solution:
[tex](90-x)\cdot 0.25=22.5-0.25x[/tex]
3. The amount of salt remains unchanged, so
[tex]22.5-0.25x=13.5[/tex]
4. Solve this equation:
[tex]22.5-0.25x=13.5\\ \\-0.25x=13.5-22.5\\ \\-0.25x=-9\\ \\0.25x=9\\ \\25x=900\\ \\x=36[/tex]
36 gallons of water need to be evaporated.
