If container a holds 8 gallons of water and container b holds 12% more than container a, how many gallons of water does container b hold?

8.96 gallons of water

To answer this question you need to multiply the container b: container a ratio with the container a volume.
Since container b has more volume than container a, the ratio should be >1. In this case, the ratio is 100%+12%= 112%
Then the volume of container b is= 112% x 8 gallons= 8.96 gallons