4 friends (Jay Leno, David Letterman, Jon Stewart and Stephen Colbert) must cross a narrow rickety bridge at night. You must use a flashlight to cross the bridge, and they only have 1 flashlight, so no more than 2 can cross at a time. Leno can cross in 1 minute. It takes Letterman 2 minutes. Jon Stewart can cross in 5 minutes but it takes Colbert 10 minutes. They only have 17 minutes for all of them to cross the bridge. How should they do it?
Weak Answer
Interviewee: "You have to send the slow guys together and then the fast guys"
Interviewer: "Are you certain about that? Talk me through how that would work in the 17 minute time constraint"
Interviewee: "I hate these kinds of problems. Can you give me a hint?"
Strong Answer
Interviewee: "Let me make sure I have the key information. (Recounts key data points)"
Interviewer: "You've got the key information right."
Interviewee: "Can one of the faster guys carry the slower guys?"
Interviewer: "No. A maximum of 2 can cross at a time."
Interviewer: "I'm going to take a minute to think about the problem and use my scratch paper. (Interviewee talks out loud as they are working on their scratch paper) I see I can't send them over individually, and I can't group the faster guys with the slower guys because there's not enough time. I think I'll need to group the two slowest guys together, but I can't send them over first because I run out of time. The solution is to send Leno and Letterman over together, have Leno come back and give the flashlight to the two slowest guys, and then have Letterman come back with the flashlight and for Leno and Letterman to cross together again.
Commentary
In the stronger answer, the interviewee asks questions to probe for non-obvious answers and talks through their analysis which helps the interviewer see that they are testing different assumptions and provide feedback if necessary.
In the Weak answer, the interviewee quickly provides an incorrect answer without any analysis. When questioned about their answer, the interviewee makes no effort to talk through other possible solutions.
This type of riddle has a correct answer, but even if you don't get to the answer in the correct time, you can get credit for structuring the problem correctly if you talk the interviewer through the approach you are taking.
Consulting firms and technology companies among others like to
incorporate questions like these into their interview process to get a
sense of how you problem solve. In some riddles you are not expected to come up with a correct answer, but to demonstrate that you can take a logical approach to solving a problem and perform basic quantitative calculations comfortably.
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