You have two empty water jugs in your hands. One holds 3 gallons of water, the other holds 5 gallons of water.
 You need to use these two jugs to measure exactly 4 gallons of water.
The answer is:  To measure exactly 4 gallons of water using a 3-gallon jug and a 5-gallon jug, you can follow these steps:

Fill the 5-gallon jug with water completely.
Pour the water from the 5-gallon jug into the 3-gallon jug until the 3-gallon jug is full, leaving you with 2 gallons of water in the 5-gallon jug.
Empty the 3-gallon jug.
Pour the 2 gallons of water from the 5-gallon jug into the 3-gallon jug.
Fill the 5-gallon jug with water again.
Pour water from the 5-gallon jug into the 3-gallon jug until the 3-gallon jug is full, which will leave you with 4 gallons of water in the 5-gallon jug.
Now you have measured exactly 4 gallons of water using the 3-gallon and 5-gallon jugs.

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