This always goes the same way:
23% of 90 L, means = 90*0.23 = 20.7 L of pure acid.
So, the equation is:
100% * L1 + 10%*L2 = 20.7, L1 is liters from 100% pure and L2 is liters from 10%
Now L1 + L2 = 90. That gives u the 2nd equation ;)
Now solve:
1*L1 + 0.1*L2 = 20.7
L1 + L2 = 90
Sure u can finish it ...