What is 80% of 500? Remember that percent means for each 100, so 80/100 = x/500. Cross multiply and we get that x = 400 gallons, so our tank has 400 gallons currently.
We can do the next step multiple ways, but let's subtract the 100 gallons to get 300. Now, 300/500 = x/100. Cross multiply again and we get that 300 gallons is indeed 60%.