I think I get this one if we view them as flow rates. Using the
MGMAT "Choose Smart Numbers" strategy you can perform the following:
If in 6 hours a tank is full, normally, then assume the tank has 60 gallons (multiple of 6). This gives:
60gal/6hrs = 10 gal/hr flow rate
Now calculate the new flow rate of the tank after it has sprung a leak. If the new time to fill the tank is 6+4 hours then t=10 and the volume remains the same:
60gal/10hrs = 6 gal/hr flow rate.
Taking the difference of flow rates you get: 10gph - 6gph = 4gph which allows you to say that the tank is leaking at a rate of 4 gal/hr.
If the tank is 2/3 full then:
(2/3)*60 = 40 gallons which defines the new volume.
To eliminate gallons and solve for the time is a simple calculation using the leaking flow rate of 4gph and the new volume:
40 gallons / 4 gal/hr =