Let's use the variable x to represent the amount of pure acid and y to represent the amount of 10% acid.
Since the total amount wanted is 90 L, we can write the equation:
[tex]x+y=90[/tex]Also, the final solution is 81%, so we can write our second equation:
[tex]100\cdot x+10\cdot y=81\cdot(x+y)[/tex]From the first equation, we can solve for y and we will have y = 90 - x.
Using this value in the second equation, we have:
[tex]100x+10(90-x)=81(x+90-x)[/tex]Solving for x, we have:
[tex]\begin{gathered} 100x+900-10x=81\cdot90 \\ 90x+900=7290 \\ 90x=7290-900 \\ 90x=6390 \\ x=\frac{6390}{90} \\ x=71 \end{gathered}[/tex]Therefore the amount of pure acid to be used is 71 L and the amount of 10% acid is 19 L.
