Question 44394
Let the amount of litres of 90% acid solution be called "x".
We know the finalsolution will have a 40% acid solution and will be 6+x litres in volume. 
Multiplying volumes by solutions, where the percentage of acid concentration is in decimal form (i.e. 0.5=50%):
{{{0.9x+0.15(6)=0.4(6+x)}}}
Now rearrange to get x:
{{{0.9x+0.9=2.4+0.4x}}}
{{{0.9x-0.4x=2.4-0.9}}}
{{{0.5x=1.5}}}
{{{x=1.5/0.5}}}
{{{x=3}}}
So 3 liters of a 90% acid solution must be added to 6 liters of a 15% acid solution to obtain a 40% acid solution.




I hope this helps.
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