1) This is a problem of rational equations, so let's set them so that we can solve it:
[tex]0.35x+0.85\left(90-x\right?=0.6*90[/tex]Note that on the left we have the mixture, 35% = 0.35 of a solution +0.85 times the number of liters we want to produce minus x. This is going to be equal to 60% (0.6) times 90.
2) Now, we can solve it:
[tex]\begin{gathered} 0.35x+0.85\left(90-x\right)=0.6\cdot \:90 \\ 0.35x+0.85\left(90-x\right)=54 \\ 0.35x\cdot \:100+0.85\left(90-x\right)\cdot \:100=54\cdot \:100 \\ 35x+85\left(90-x\right)=5400 \\ -50x+7650=5400 \\ -50x=-2250 \\ \frac{-50x}{-50}=\frac{-2250}{-50} \\ x=45 \end{gathered}[/tex]Once the hardest part is done, we had to deal with the good old Algebra.
