Answer
$96$ gallons
Work Step by Step
Let $y$ be the amount of water that has evaporated.
There are $0.03\cdot240=7.2$ gallons of salt.
Then our equation is: $\frac{7.2}{240-y}=0.05\\7.2=0.05(240-y)\\7.2=12-0.05y\\0.05y=4.8\\y=96$
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