You need 490 mL of a 40% alcohol solution. On hand, you have a 70% alcohol mixture. How much of the 70% alcohol mixture and pure water will you need to obtain the desired solution?

Answer 1

0.40 * 490 = 196 mL of pure alcohol needed
196 mL = 0.70 * x mL of 0.7
x = 280 mL of 70%
490 - 280 = 210 mL of H2O