Riddle For Your Enrichment & Possibly Amusement
Posted by Rob Herman at March 14th, 2007
This is an old riddle that many puzzle fans will have heard before. It’s not new to me, and I’m not a big fan of it.
But, a friend of mine was recently posed this brainteaser during a technical interview, and she hadn’t heard it before. As a public service, I now re-present to you this old riddle, that you may ace your next interview.
You have two jugs of water. One holds three gallons and one holds five gallons, though they are otherwise unmarked. You also have a sink that you can get water out of or dump excess water into. Your task is to measure out exactly four gallons of water.
For the record, I am opposed to using this kind of question on an interview; it is 90% “have you heard this question before”, 8% sangfroid, and no more than 2% of whatever mental ability the interviewer is hoping to measure.
Well, I heard the riddle before, but not the word sangfroid. I looked up the definition, so now I have be educated.
At the risk of sounding undeservedly pompous, isn’t this riddle easy enough to be a fine measure of some basic reasoning skills, so long as they don’t answer from memory.
To be honest, if I were the interviewer I’d be using this question as a way to read how the person will react to unexpected situations. Also, I’d be looking to see if the person had ‘class’ when feeling confronted.
This is a dead giveaway that I’ve seen some awful movies, but I’m pretty sure in ‘Die Hard 3: With A Vengeance’, there was a bomb could only be disarmed by solving this very riddle and placing the resulting jug of water on a scale that disabled the explosive. It seemed strange to me that someone with multi-million dollar funding could devise such a complicated explosive and triggering mechanism, but couldn’t google for a riddle that everyone hadn’t already heard.
I think this shows up before Jeremy Irons is actually supposed to be killing Bruce Willis, so using a well-known and relatively simple riddle is actually somewhat sensible.
Also it would totally blow credibility to have any of those characters solving a difficult riddle. They’re much more likely to take a 90 foot fall without injury.
In theory, were I asked this at an interview and thinking clearly, rather than firing off the final answer, I would build the entire logical foundation for the solution. At worst, I would expect this to demonstrate a depth of understanding of the solution, with the potential side effect of appearing to solve spot-on. Unless the interviewer asks directly whether I’ve heard it before.
Rule0 - serving the public since 2007.
Alatar: Yeah. I certainly wouldn’t volunteer the fact that I had seen the riddle before. I would collect my thoughts to make sure I remembered the solution correctly, then explain it, something like:
“Well, we’ll need to isolate one gallon so we can add three gallons to it and make four. How do we do that? Well, fill the three gallon jug and dump it into the five gallon jug. Then fill up the three gallon jug completely and fill up the five gallon jug–that leaves one gallon in the three gallon jug. Empty the five gallon jug and dump the single gallon into it, then add another three gallons from the three gallon jug to make four.”
MJB: An interview situation is so high pressure that I wouldn’t be surprised to see a person that could solve it on any other day of the week choke entirely.
I would rather ask a question that doesn’t have a binary, right-or-wrong outcome. After all, hardly anything I ever work on does. Heck, say, “If you were designing a module to do XYZ, what would your considerations be?” This gives you insight into their thought process, creativity, and also experience.
Granted, if I were the interviewer, this wouldn’t be one that I would ask… I guess I was only trying to figure out what the perceived benefit of asking such a ‘riddle’ of a question.
Perhaps they are asking such a question because they assume most people have heard it, or can figure it out easily. Instead of looking for a ‘right’ answer, the interviewer is looking for a ‘wrong’ answer. Thus showing either an inability to think analytically, or an inability to stay calm.
It is possible that instead of looking at a right answer as a positive, they are simply looking for a ‘wrong’ answer as a negative.
I’m pretty sure that the following also works and heh, if asked this question in an interview, it might be better than the easy answer.
If the vessels are right cylinders, then if you pour water out of the vessel until the plane of the water intersects the circumference of the can both at the top and the bottom, then the can will be half-filled with water. If you do this with both cans, then one will contain 1.5 cups of water and the other will contain 2.5 cups of water. Pour the 1.5 cups into the 2.5 cups and you have a total of 4 cups of water.
“I’ve seen something kind of like this before.
“Let’s see, to end up with 4 gallons in the 5 gallon jug the next to last step is we have 1 gallon in the 5 jug and we pour in 3 more. Or we have 2 gallons in the 3 jug and we pour one from the 5 leaving 4.
“We can’t pour four gallons out of the 5 jug to get one, so we need to pour 2 gallons out of the 3 jug to get one. Looks hard.
“Try the other way. To get 2 gallons in the 3 jug first get 2 gallons in the 5 jug. We need to pour off 3 gallons. We can do that!
“So that’s the answer. Fill the 5 gallon jug and pour off 3 gallons. Empty the 3 gallon jug and then empty the 2 gallons from the 5 gallon jug into it. You’ll have 2 gallons in the 3 gallon jug. Fill the 5 gallon jug and then pour it into the 3 gallon jug, one gallon will go into the 3 gallon jug and that leaves 4.”
I’ve never ever been asked a question like this on a job interview. By the time it gets to the interview stage they know I can do that sort of thing.
The kind of question I’m afraid of goes like:
You’re working on a project where you have to cooperate with 3 other people. Two of them disagree about politics — one of them supports the iraq war and the other one thinks that Bush never tells the truth about anything. They get so mad at each other that you’re afraid they might get violent. What do you do?
Was that friend me, or another friend who faced the same thing?
I figured it out on the drive home, called him back and told him the answer. And later got the job.
Yeah, M, that was you. Congratulations!