15% of 20 gal = 0.15 * 20 gal = 3 gal, so the solution contains 3 gal of salt.
If we add x gal of water to the solution, we end up with (20 + x) gal of solution. We want the new mixture to have a concentration of 10%, or
10% of (20 + x) gal = 0.1 * (20 + x) gal = 2 + 0.1x gal
of salt.
The amount of salt in the tank hasn't changed. Solve for x :
2 + 0.1x = 3
0.1x = 1
x = 10 gal
