Solution Concentration Problem: A Mental Math Weekend Challenge

A Weekend Mental Math Challenge

Here’s your weekend puzzle from the mental math series. Try to solve it in your head first — then share your answer and how it felt in the comments!

The Solution Concentration Problem

A vessel contains 4 liters of an 8% solution. 12 liters of water are added to it. What is the concentration of the substance, in percent, in the resulting solution?

The Answer and Where This Problem Comes From

The answer is 2%.

Here’s the logic: the original vessel contains 4 × 0.08 = 0.32 liters of pure substance. After adding 12 liters of water, the total volume becomes 16 liters. The new concentration is 0.32 ÷ 16 = 0.02, or 2%.

This solution concentration problem, by the way, appeared on the Russian national math exam (EGE) — in the second section — a few years back.