Graduate Management Admission Test (GMAT) Practice Test

When two liquids are mixed, if liquid A is 5% and liquid B is 20%, which would you need in a greater part to achieve 15%?

1. Liquid A

2. Liquid B

3. Equal parts of both

4. Neither, both are excessively concentrated

The correct answer is: Liquid B

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