Answer:
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 [tex]6\cdot 0.15=0.9[/tex] liter of acid in 6 liters of 15% acid solution
There are [tex]x\cdot 0.9=0.9x[/tex] liter of acid in x liters of 90% acid solution
There are [tex](6+x)\cdot 0.4=0.4(6+x)[/tex] liter of acid in 6 + x liters of 40% acid solution
So,
[tex]0.9+0.9x=0.4(6+x)[/tex]
Solve this equation
[tex]9+9x=4(6+x)\\ \\ 9+9x=24+4x\\ \\9x-4x=24-9\\ \\5x=15\\ \\x=3[/tex]
