Heath needs to end up with 10L of solution,
of which 9L is bleach. So let m = liters of 20%
solution needed. Then .20m liters of that are bleach.
He must then add p liters of bleach to make
a total of 10 L. That is, p + m = 10. So the system is
m + p = 10, .20m + p = 9. Solving,
m = 1.25 L 20% solution, and thus p = 8.75 L bleach.