GMAT Practice Test – Full Prep, Study Guide & Practice Questions

To achieve a final concentration of 15% by mixing two liquids with differing concentrations, it is essential to understand the concept of weighted averages. Liquid A is at 5% concentration, while Liquid B is at 20% concentration. The target concentration of 15% lies between these two percentages.

To determine how much of each liquid is needed to reach the desired concentration, we can use a concept known as the method of alligation, which visually helps to understand the proportions of the mixtures. The difference between the lower concentration (5%) and the target concentration (15%) is 10%, while the difference between the higher concentration (20%) and the target concentration (15%) is 5%. This informs us about the relative amounts needed of each liquid.

In practical terms, to reach a middle concentration like 15%, more of the liquid with the higher concentration (Liquid B) must be added in order to dilute the overall mixture down from 20% to 15%. Thus, to balance the lower and higher concentrations until the average is at 15%, a larger volume of Liquid B is required compared to Liquid A.

This principle shows that in order to reach a specific target concentration when mixing liquids of different concentrations, you have to assess not