Question 665032
Let the amount of pure acid be given by x. Then if the total volume is 90 liters, the amount of 10% acid is given by (90-x). Plug that in and solve
100%x + 10%(90-x) = 89%(90)
{{{x + 0.1(90-x) = 0.89 * 90}}}
{{{x + 9 - 0.1x = 80.1}}}
{{{0.9x = 71.1}}}
{{{x = 79}}} so there must be 11 liters of 10% to start with.
And remember .... add acid to water, just like you oughta (and not the other way around)