You have two buckets - one holds exactly 5 gallons and the other 3 gallons. How can you measure 4 gallons of water into the 5 gallon bucket?
(Assume you have an unlimited supply of water and that there are no measurement markings of any kind on the buckets.)
[toggle title="What's the answer?"]Fill the 3-gallon bucket.
Pour the 3 gallons of water into the 5-gallon bucket
Fill the 3-gallon bucket again.
Fill up the 5-gallon bucket with the 3-gallon bucket, leaving you with 1 gallon left in the 3-gallon bucket.
Empty out the 5-gallon bucket.
Pour the remaining 1 gallon of water from the 3-gallon bucket into the 5-gallon bucket.
Fill the 3-gallon bucket.
Pour the 3 gallons of water from the 3-gallon bucket into the 5-gallon bucket leaving you with 4 gallons of water in the 5-gallon bucket.
Fill up the 5 gallon bucket
Pour it into 3 gallon bucket, leaving 2 gallons
Empty out the 3 gallon bucket
Pour the 2 gallons in the 5 gallon bucket into the 3 gallon bucket
Fill up the 5 gallon bucket and pour it into the 3 gallon bucket until it's full, leaving 4 gallons in the 5 gallon bucket.[/toggle]