Randall has a fish tank that can hold up to 30 gallons of water but it is leaking at a constant rate. When Randall first notices the leak, the tank has 21 gallons of water. Two hours later the tank has 18 gallons. How many hours BEFORE Randall noticed the leak was the fish tank completely full? 1) 2 hours
2) 6 hours
3) 4 hours
4) 8 hours