How much pure acid should be added to 36L of a 15% acid solutuion to make a 40% acid solution

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