The tank is leaking at the rate of 6 gallons per hour.
If you do nothing but sit there and watch it all afternoon,
here's what you'll see:
First hour . . . . 6 gallons leaks out
Second hour . another 6 gallons leaks out
Third hour . . . another 6 gallons leaks out
Total gone after 3 hours: (6 + 6 + 6) = 18 gallons .