Question 160147
Let x=amount of water that needs to be evaporated

Now we know that the amount of pure salt in the 240 gal (0.03*240) has to equal the amount of pure salt in the solution after the evaporation takes place(0.05(240-x)).  So our equation to solve is:

0.03*240=0.05*(240-x)  get rid of parens and simplify
7.2=12-0.05x  subtract 12 from each side
7.2-12=12-12-0.05x  collect like terms
-4.8=-0.05x divide each side by 0.05
x=96 gal---------------------------------amount of water that needs to be evaporated

CK
0.03*240=0.05*144
7.2=7.2

Hope this helps---ptaylor