How many gallons of a 5% bleach solution must be added to 10 Gallons of a 20% bleach solution to
produce that is 15% bleach

Step-by-step explanation:

10 gallons of the 20% solution contain 2 gallons of bleach.

x gallons of the 5% solution contain 0.05x gallons of bleach.

If the two are combined, the resulting 10+x gallons contain 2+0.5x gallons of bleach. The concentration is (2+0.05x)/(10+x).

(2+0.05x)/(10+x) = 15%

2+0.05x = 0.15(10+x)

2+0.05x = 1.5+0.15x

0.5 = 0.10x

x = 5

5 gallons of 5% solution is added to the 10 gallons of 20% solution.