Google Brain-teaser

Tim Challies posted a link this morning to “15 Google Interview Questions That Will Make You Feel Stupid.” It turns out that Google is hiring again, and they like to ask hard questions in their interviews. In fact, you might say that their interview questions are like taking the GRE. Here’s one of the brain-teasers they use when interviewing product managers. See if you can figure it out before looking at my answer in the comment section. Let me know if you got it right.

You have eight balls all of the same size. Seven of them weigh the same, and one of them weighs slightly more. How can you find the ball that is heavier by using a balance and only two weighings?

8 Responses to Google Brain-teaser

  1. Denny Burk November 20, 2009 at 11:43 am #

    1st Weighing
    Take 6 of the balls and weigh them 3 on one side and 3 on the other.

    2nd Weighing
    If both sides of three are equal, then weigh the remaining two balls to see which one is heavier.

    If one set of three is heavier than the other set of three, then select two balls from the heavier set to weigh.

    If one of the two is heavier than the other, then you’ve found your ball.

    If the two balls are equal, then you know that the third unweighed ball is the heaviest.

  2. Joshua November 20, 2009 at 12:45 pm #

    This is a classic Computer Science question. I came across it three times at my last job (Meditech, not Google). So, yes, now I finally get this question right 🙂

  3. Doug November 20, 2009 at 1:46 pm #

    Very good grasshopper. My brain got tired trying to figure it out.

  4. Mark November 21, 2009 at 1:49 am #

    Yeah. My mind isn’t supposed to work with these types of questions.

  5. Alex Chediak November 21, 2009 at 2:47 pm #

    My brother-in-law works for Google. He can attest to the challenge of their interviews!

  6. jeff miller November 21, 2009 at 7:34 pm #

    I got it. Stone is my witness 🙂

  7. Don Johnson November 22, 2009 at 4:18 pm #

    You can even do it with 9 balls.

  8. Peter Sewell November 22, 2009 at 11:09 pm #

    “their interview questions are like taking the GRE.” ha ha ha ha ha

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