Answer: He should add 8 gallons of water to make 5% bleach solution.
Step-by-step explanation:
Let the number of gallons of water he should add be 'x'.
Since we have given that
Total number of gallon we have = 8
It has 10% beach solution i.e. 
We need to make it a 5% bleach solution by adding water to it.
so, our equation becomes,

Hence, he should add 8 gallons of water to make 5% bleach solution.