So we start out by finding out what 80% of 500 is by,
500 * .80 = 400
So if 80% is 400 that means that there is still space available for 100 more gallons!
How the heck did we know that?!?
simple 500- 400= 100
So that leaves us knowing that there are 100 gallons left that can be held!! :)
So your answer is 100 gallons!!
I hope that this helped!