Answer
Approximately 3.06 gal
Work Step by Step
x will be the amount of solution replaced by pure bleach
We start with 100gal (2% concentration=0.02) and we should end up with 100 gal (5% concentration = 0.05).
0.02(entire amount minus amount to be replaced)
0.02(100-x)
1x is the amount of pure bleach
0.05(100) is result
0.02(100-x)+1x =0.05(100)
2-0.02x+x =5
0.98x=3
x=$\frac{3}{0.98}$ $\approx$ 3.06
