A water tank holds 18,000 gallons. How long will it take for the water level to reach 6000 gallons if the water is used at an average rate of 450 gallons per day?

You start with 18.000 gallons and want to know when you reach 6.000 gallons, with a daily use of 450 gallons. This means you need to know how many days it take to use 18.000 - 6.000 = 12.000 gallons. This can simply be calculated with 12.000/450, which gives 26 2/3 day