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Question

A lab technician mixed a 690 ml solution of water and alcohol. If 3% of the solution is alcohol, how many milliliters of alcohol were used, and how many milliliters of water were used

Answer 1

Assuming that the alcohol is 3% by volume (and not by mass), then you have 3 ml alcohol/100 ml solution.

3 ml/100 ml x 690 ml = 20.7 ml alcohol and 690 - 20.7 ml = 669.3 ml water (assuming the volumes are additive, which they are not when mixing alcohol and water).  Since this is a math/algebra problem and not a chemistry problem, the concerns about additivity can probably be ignored.