To get the amount of water you would add to 2.5 gallons of bleach to make 10% bleach we shall proceed as follows:
let the amount of water be x
100*2.5+0x=10(x+2.5)
250=10x+25
10x=225
x=22.5
thus the answer is, 22.5 gallons of pure water should be added in the bleach.