ANSWER
23/24 of the tank is filled with water
Step-by-step explanation
The tank can hold 800 gallons of water and you have 3/4 full:
The tank has 600 gallons of water. Then you drain 100 gallons, so now you have 500 gallons of water in the tank.
Then, you add 1/3 of a tank. If one tank can hold 800 gallons, 1/3 of a tank is:
Therefore, you have 266 2/3 plus 500 gallons you had before:
To find what fraction of the tank that represents we have to divide it by 800:
23/24 of the tank is filled with water.