Logic Puzzles

2. Four Gallons

You have a three gallon and a five gallon measuring device. You wish to measure out four gallons.

Added 1 January 2007 · Updated 5 July 2026

Solution:

Fill the five gallon container. Pour all but two gallons into the three gallon container. Empty the three gallon container. Put the two remaining gallons from the five gallon container into the three gallon container. Fill the five gallon container one more time. Pour one gallon from the five gallon container by filling the three gallon container. Now the five gallon container contains four gallons.

I have an alternate answer for the Four Gallons problem. Fill up the 3-gallon and pour it into the 5-gallon. Again, fill up the 3-gallon and pour it into the 5-gallon. Now, 1 gallon will be left in the 3-gallon. Pour out all the water from the 5-gallon and fill it up with that one gallon. Now, measure 3 gallons and pour it into the 5-gallon to bring out 4.


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Comments (31)

Anonymous 25 June 2007

The solution to the Four Gallons puzzle is incorrect. The correct method is to fill the 3-gallon container and pour it into the 5-gallon container, then refill the 3-gallon container and pour again until the 5-gallon container is full, leaving 1 gallon in the 3-gallon container.

Anonymous 13 July 2007

The problem of measuring four gallons has another solution. Measure 3 gallons and pour into the 5 gallon device. Measure 3 gallons again and pour into the 5 gallon device. You are left with 1 gallon in the 3 gallon device. Empty the 5 gallon device, pour the remaining 1 gallon into the 5 gallon device, measure 3 gallons and pour into the 5 gallon device, measuring a total of 4 gallons.

Anonymous ★ Solved 8 January 2009

I have an alternate answer for the Four Gallons problem. Fill up the 3-gallon and pour it into the 5-gallon. Again fill up the 3-gallon and pour it into the 5-gallon. Now 1 gallon will be left in the 3-gallon. Pour out all the water from the 5-gallon and fill it up with that one gallon. Now measure 3 gallons and pour into the 5-gallon to bring out 4.

Anonymous 18 March 2009

I have found a much more simple answer to the puzzle "Four Gallons": Since the question does not specify the type of containers we have, we assume that we have a container for any chosen number of gallons. Measure 5 gallons into a 5 gallon container. Do this one more time. Now, mix these into one 10 gallon container. Take away 6 gallons by filling two 3 gallon containers from the 10.

Anonymous 18 March 2009

I have found a simpler answer to 'Four Gallons': Measure 5 gallons into a 5-gallon container twice, mix into a 10-gallon container, then fill two 3-gallon containers from it to take away 6 gallons, leaving 4 gallons.

Anonymous 29 March 2009

For number 2 on the easy logic problems, isn't there more than one answer? You can also fill up the three gallon, pour it into the five gallon, then fill up the three gallon again and fill the five gallon so there is one gallon left in the three gallon. Then empty the five gallon, pour the one gallon from the three gallon container into the five gallon, then add three gallons from the three gallon.

Anonymous 6 December 2009

In the solution to #2 Four Gallons, if you pour all but two gallons from the 5-gal. container then the container itself must show graduations of measurement. One would simply pour 4-gal. into the container.

Anonymous 10 February 2010

There is another way to solve Four Gallons: fill the three gallon container, pour the three gallons into the five gallon container, fill the three gallon container again and pour two of them into the five gallon container leaving one gallon in the three gallon container. Empty the five gallon container and pour the one gallon from the three gallon container into it.

Anonymous 7 April 2010

Four Gallons can also be solved this way. Fill three and pour into five. Fill three and fill five with it, leaving one in three. Empty five, pour the one in three into five, then fill three and pour into five, giving four.

Anonymous 7 April 2010

Four Gallons can also be solved this way: fill the three gallon and pour into the five. Fill the three gallon again and fill the five with it, leaving one in the three. Empty the five, pour the one in the three into the five, then fill the three and pour into the five, giving four.

Anonymous 7 April 2010

The solution to Four Gallons can also be approached by filling the three-gallon jug and pouring it into the five-gallon jug. After that, refill the three-gallon jug and pour into the five-gallon jug until it is full, leaving exactly one gallon in the three-gallon jug.

Anonymous 19 April 2010

A more accurate answer for four gallons would be: fill the three gallon then tip into the five gallon, fill the three gallon again then tip into the five gallon to top, leaving exactly one gallon in the three gallon bucket. Tip out the five gallon bucket and tip in the one gallon from the three gallon bucket, finally fill the three gallon and tip into the five gallon.

Anonymous 19 April 2010

The solution for question 2 regarding the four gallons is more accurate as it specifies the steps clearly. Fill the 3-gallon bucket, pour into the 5-gallon bucket, and repeat to achieve exactly 4 gallons.

Anonymous 24 February 2012

The answer to the puzzle is 90.

Anonymous 30 March 2012

There is an alternative solution to the 4 gallons problem. You can fill the 3-gallon bucket, pour it into the 5-gallon bucket, and repeat the process to achieve the desired amount.

Anonymous 28 April 2012

For the four gallon question, there is a different solution: fill the three gallon, dispense into the five gallon, fill the three gallon again, dispense into the five gallon until full, at which point you have one gallon in the three gallon container, then empty the five gallon, dispense the remaining one gallon into the five gallon, fill the three gallon one final time and use that to add the one gallon in five, making four gallons.

Anonymous 28 April 2012

For the four gallon question, there is a different solution: fill the three-gallon container, pour into the five-gallon until full, then repeat to achieve four gallons.

Anonymous 11 September 2012

I agree with the 3 gallon and 5 gallon but there are 2 solutions. You can fill the 3, pour into the 5, refill the three, pour into the five.

Anonymous 26 September 2012

There is another answer to the Four Gallons question. If the buckets are not irregularly shaped, you can fill both by half to make 4 gallons.

Anonymous 21 October 2012

If I can measure 3 gallons in it, then I can obviously measure 4 gallons in it. I wouldn't go through the elaborate procedure given as the solution.

Anonymous 21 October 2012

Regarding the four gallons question, I believe I can measure 4 gallons by first measuring 3 gallons, which seems simpler than the provided solution.

Anonymous 26 October 2012

The solution to No. 2 in easy logic problems implies knowledge of the two-gallon mark, which is not provided. The correct answer should only use the knowledge given of the three and five-gallon containers.

Anonymous 26 November 2012

I think there is no error in this puzzle. It is a good questionnaire that will be enjoyable for participants.

Anonymous 18 March 2013

The solution to measuring four gallons using a three-gallon and a five-gallon container is simpler than described. Fill the three-gallon container and pour it into the five-gallon container. Repeat this process to measure out four gallons.

Anonymous 7 May 2013

The solution for the four gallons puzzle is slightly less efficient than the one I came up with. My solution utilizes only 9 gallons.

Anonymous 19 May 2013

Your solution to the 4 gallons problem is flawed. You say to fill the five gallon container and pour all but two gallons into the three gallon container. However, if I could measure 2 gallons in the five gallon container, I could measure 4 by doubling it.

Anonymous 14 July 2014

The puzzle FOUR GALLONS solution is incorrect. How would you measure out two gallons in the first place if you only have a five and three gallon measuring buckets? A better solution is [...]

Anonymous 4 March 2016

Four Gallons Alternate solution: Fill the 3 gallon, pour it into the 5 gallon, fill the 3 gallon again, and fill the rest of the 5 gallon. One gallon is left in the 3 gallon.

Anonymous 14 March 2017

In question 8, you need to count one point twice. The answer remains the same regardless of whether the boy is younger or older.

Anonymous 14 July 2017

You can solve the four gallons of water puzzle without wasting extra water. Just fill both buckets halfway.

Anonymous 14 July 2017

The solution is even more simple: fill the 3 gallon container, pour it into the 5 gallon container, and repeat to completely fill the 5 gallon container.

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