A tank holds a 40% sugar solution. Some of it is drained off and replaced with the same amount of a 16% sugar solution. The tank now holds a 25% solution. What fraction of the solution was replaced?

Explanation

1. 01

   After replacement the tank is a mix of old 40% solution and new 16% solution, averaging 25%.

2. 02

   Old : new = $(25 - 16) : (40 - 25) = 9 : 15 = 3 : 5$.

3. 03

   Replaced fraction = $\frac{5}{3 + 5} = \frac{5}{8}$.