If you add x gal of the 18% mixture, each gallon adds 0.18 gal of salt, so x gal would add 0.18x gal of salt.
The 15 gal of 2% salt mixture already contains 0.02 (15 gal) = 0.3 gal of salt.
The total volume of the new mixture would be (15 + x) gal. To achieve a 7% concentration of salt, you would need to add x gal of the 18% mixture such that
(amount of salt) / (total volume) = (0.3 + 0.18x) / (15 + x) = 0.07
Solve for x :
0.3 + 0.18x = 0.07 (15 + x)
0.3 + 0.18x = 1.05 + 0.07x
0.11x = 0.75
x = 0.75 / 0.11 ≈ 6.82 gal
