You have a 5 gallon bucket, a 3 gallon bucket, and a hose to fill them up. You need to get 4 gallons. You have no means of measuring how many gallons are in each bucket (except knowing the bucket capacity). How can you be certain that you have 4 gallons?
Fill up the 3 gallon bucket and pour it in to the 5 gallon bucket (You now have 3 gallons in the 5 gallon bucket). Fill up the 3 gallon bucket again and pour it into the 5 gallon bucket until it is full (You now have 5 gallons in the 5 gallon bucket and 1 gallon in the 3 gallon bucket). Dump the water out of the 5 gallon bucket. Pour the one gallon from the 3 gallon bucket into the 5 gallon bucket (You now have 1 gallon in the 5 gallon bucket). Fill up the 3 gallon bucket and pour it into the 5 gallon bucket. You now have 4 gallons in the 5 gallon bucket.
SEE ANSWER