How many liters of a 90% acid solution must be added to 6 liters of a 15% acid solution to obtain a 40% acid solution?

3 liters

Step-by-step explanation:

Let x liters of 90% acid solution is added to 6 liters of a 15% acid solution to obtain a 40% acid solution.

There are liter of acid in 6 liters of 15% acid solution

There are liter of acid in x liters of 90% acid solution

There are liter of acid in 6 + x liters of 40% acid solution

So,

Solve this equation