Answer: To achieve a 5% bleach solution, he needs to add 8 gallons of water.
Step-by-step explanation:
Let 'x' represent the gallons of water he is meant to add.
We know that
The total current volume is 8 gallons
Containing a 10% bleach solution, which is i.e. 
The goal is to dilute it to a 5% bleach solution by incorporating additional water.
Consequently, the equation can be defined as,

Therefore, he should add 8 gallons of water to create a 5% bleach solution.