work in volumes of pure acid
Vol pure acid in 15% solution = 36*0.15 = 5.4 L
Volume of pure acid when x L of pure acid has been added = 0.40(x + 36)
so x + 5.4 = 0.40(x + 36)
x + 5.4 = 0.40x + 14.40
0.60x = 14.4 - 5.4 = 9
x = 9 / 0.6 = 15 L
Answer is 15 liters
