The gauge of an oil tank indicated that the tank was 1/7 full.  After 240 gallons of oil were added to the tank, the gauge indicated that the tank was 4/7 full.  How many gallons of oil does the tank hold, assuming that the gauge is accurate?



560 gallons

Since 3/7 of the tank, or 240 gallons, was added, then 1/7 of a tank holds 80 gallons.  The whole tank holds 7 times 80, or 560, gallons.