Please do NOT post your answer, but do say if you think you have solved the puzzle and how long it took. Solution on Monday.

I have a 100 pound watermelon laying out in the sun. 99% of the watermelon’s weight is water. After a few hours 98% of the watermelon’s weight is water. How much water evaporated?

I have produced an ebook containing 101 of the previous Friday Puzzles! It is called PUZZLED and is available for the Kindle (UK here and USA here) and on the iBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.


    1. Ian, I agree – I had to double check my workings … 1% is such a small change. I had a calculator out at first in preparation for decimal places.

    2. This took me longer than I care to admit. After initially reading it, I decided that I needed to get breakfast and coffee before I wanted to deal with it. Then I looked at it again and it looked unpleasant so I took a walk. Then I came back and realized that it was really easy and from there it took me about thirty seconds. So about two hours.

    3. Wow. I couldn’t figure it out quickly and so did the formal calculation only to find how far off the mark I had been. Of course, it’s all so easy to see in retrospect. Nice one for leading me astray.

    1. About 30 mins for the answer I have. Took a little while to determine how to work it out. Then a few guesses on the calculator and I had my answer.

  1. about 2 minutes – including pondering how to approach it, then jotting down some algebra, pondering the answer and considering if it was sensible, checking my working and deciding I’d got a valid answer. I could still be wrong though.

  2. This is very clever but we are not given enough information to answer the puzzle.

    After a “few hours” the weight of a watermelon may have changed in many ways. Perhaps it has been sprayed by insecticide. It will have grown some more seeds. Perhaps it has ingested a slug.

    Even if we research the growth patterns of a watermelon as perhaps Richard wants us to do we still do not have enough information because we do not know the energy budget. IE how hot the is shining or what chemical nutriments are available in the soil.

    It is not uncommon for prize vegetable growers to use growth hormones and such the like on their prize fruits. That adds another dimension especially if they have cheated by injecting gycerole or other substances.

    Puzzles should be challenging not impossible.

    1. Careful – your parodies of Barry are getting so good they are beginning to sound like the real Mr Goddard!

    2. Thank you @mathmiles for your support.

      It is completely obvious that Richard intends that it is a watermelon to be part of the puzzle. A watermelon is a living breathing vegetable – a fauna in technical botany terms.

      If Richard had said “I have a bucket that wieghs one pound with 99 pound water in it. What percentage of water remains if I empty 50 pounds out?” That would be the puzzle that most people here appear to be solving.

      Only those of us who respect the question and read it properly will be directed to the deeper answers.

      My experience in producing star charts where precise readings are melded with intuitive insights helps me go further like this.

  3. I’ve got an answer but I doubt it’s right. I did it on paper with a bit of Heath Robinson algebra. Couple of minutes.

    1. The glass is always full and the sound of one hand clapping is the sound of one hand clapping.

  4. a. 100-pound watermelon?! That is enormous, unless…
    b. 100-pound watermelon?! That is expensive!
    c. Okay, who cut into the watermelon to just have a bite?!

  5. It took me a few seconds to realise that as the water evaporated, the overall weight of the watermelon also reduced. One quick fill down in a spreadsheet later, and I had my answer (although given what the answer is, there’s probably a far quicker algebraic method).

    That is, of course, assuming water is the only component of the melon’s composition which reduces in weight over time…

  6. This should help people figuring it out, initially 1% is not water and this 1% equals 1 pound. Later 1 pound is still not water but now this 1 pound is 2%.

  7. 20 seconds of head calculations and then two minutes to read the comments to find confirmation —- but this time it was wasted time since there is no answer — yet. Whats going on here,

  8. From Wikipedia…
    A watermelon contains about 6% sugar and 91% water by weight….
    The more than 1200 varieties of watermelon range in weight from less than one to more than 200 pounds; the flesh can be red, orange, yellow or white…

    I’d never have thought to expand my knowledge about watermelons if this puzzle had never been posed. Thank you Richard.

  9. I agree it’s a great puzzle. I had to use a pen to write down an equation.
    Then it made sense. Took maybe 4 minutes partly to scratch my head when I got the answer.

  10. Despite my three years of Sixth grade, I am floundering again between what was, what is…and what may ever be.

  11. about 4-5 minutes scribbling the calculation on a bit of scrap paper with a pencil. Then deriving a proper algebraic expression to be certain it was correct.

  12. About 5 minutes to get the wrong answer, find my math error, and end up with an even more surprising (and correct) result.

    One of the best “puzzles” you’ve had in a long while!

  13. It took me about 10 seconds to get the obvious and correct answer, then several minutes getting confused by reading the thread and seeing the comments. Lots of people seem to be giving lots of strange cryptic clues which suggest that they find it more confusing than it actually is.

    1. It’s easy to say it in math.

      m_1=100lb\\ r_1=99\%\\ r_2=98\%\\
      Find out

      System of equations is translated from given conditions.
      \begin{cases}    m_{w1} = m_1 * r_1 (1)\\    m_{w2} = m_2 * r_2 (2)\\    m_2 = m_1 - m_{e-w} (3)\\    m_{e-w} = m_{w1} - m_{w2} (4)\\   \end{cases}
      Solution for system of equations is:
      m_{w2} = (m_1 - m_{e-w}) * r_2 (5) (from 2, 3) => \\ m_{e-w}= m_1*r_1 - (m_1 - m_{e-w})*r_2 (from 5, 1, 4) => \\ m_{e-w} = m_1*r_1 - m_1*r_2 + m_{e-w}*r_2 => \\ m_{e-w} * (1-r_2)= m_1 * (r_1 - r_2) \\

      Final result:
      m_{e-w} = m_1 * \frac{r_1 - r_2}{1 - r_2}
      Add numbers:
      m_{e-w} = 100 * \frac{0.99 - 0.98}{1 - 0.98} = 100 * \frac{0.01}{0.02} = 100 * \frac{1}{2} = 50 lb

      Any questions?

    2. Fifty pounds isn’t the correct answer anyway, though it shows that you figured out how to solve it. You just didn’t finish solving it.

  14. It took 30 sec to figure this one out. The result is surprising, although the calculation is absolutely correct.

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