Fire it up!
Your entire tour group has been shipwrecked and is now stranded on a desert island.
You managed to salvage an oil lamp, and three pots of oil.
One of the pots contains seven gallons of oil, the other two contain five gallons each.
Upon closer inspection, you notice that the seven-gallon pot is full, while the other two are empty.
You would like to light the oil lamp at night to signal passing ships.
Fortunately, your island is close to shipping lanes.
You now know that there is a four-hour window in which the lamp can be lit.
There is always a cruise ship that passes by between 10:00 PM and 2:00 AM, and if your lamp is lit when it passes, a rescue helicopter is sure to arrive the next day.
-A gallon of oil burns for exactly one hour.
-If you pour off a smaller amount into a five-gallon pot, you may not have enough fuel to last the full four hours.
What do you need to do in order to end up with exactly four gallons of oil in one of the pots?