Word Problem: Increasing Solution Strength to 30%

[Diced Tofu]

1. A tank holds 90 L of chemical solution. Currently the solution has a strength of 20%. How much of this should be drained and replaced with an 80% solution to have a final strength of 30%?

 

[HallsofIvy]

Suppose you "drain and replace" X liters of the 20% solution. That leaves 90- X liters of 20% solution. The X liters of the 80% solution will contain 0.8X L of the chemical. The remaining 90-X liters will contain 0.2(90- X) L of the chemical so you will now have 0.8X+ 0.2(90-X) L of the chemical in the final result. How much chemical will there be in 90 L of 30% solution? Set those equal and solve for X.

 

[Diced Tofu]

Answer is 15 L.

thx :D