Question

You need 270 mL of a 95% alcohol solution. On hand, you have a 55% alcohol mixture. How much of the 55% alcohol mixture and pure alcohol will you need to obtain the desired solution?

Answer 1

Let m be the amount in mL you need of the 55% mixture. The amount of alcohol in the mixture is 0.55mL.

Let a be the amount in mL you need of the pure alcohol. We need 270mL altogether, so a+m=270.

The amount of alcohol in the final solution is 0.95×270=256.5mL.

So 0.55m+a=256.5. That is, 0.55m+270-m=256.5. Therefore, 0.45m=270-256.5=13.5. m=13.5/0.45=1350/45=30mL.

So we need 30mL of the mixture on hand and 270-30=240ml of pure alcohol.