Alcohol mixture problem

Question

You need a 30% alcohol solution. On hand, you have a 315 mL of a 25% alcohol mixture. You also have 65% alcohol mixture. How much of each mixture will you need to add to obtain the desired solution?

You will need

 ___ mL of the 65% solution

to obtain

___ mL of the desired 30% solution.

Answer 1

Let A be the amount of 65% alcohol solution that must be added.

315mL x 0.25 + 0.65A = (315 + A) x 0.3

78.75 + 0.65A = 94.5 + 0.3A

0.35A = 15.75

A = 45mL

The total amount of 30% alcohol solution will be 360mL.