A mixture of 10% acid and 90% water is added to 5 liters of pure acid. The final mixture is 40% water. How many liters of water are in the final mixture?

The total amount of water, divided by the total amount of acid/water mixture is 40%. We then solve for the amount of mixture that we had prior to adding the 5 L of pure acid.

.9x / (x + 5L) =.4

.9x =.4x + 2 L

.5x = 2L

x = 4L

You started with 4 Liters of the mixture