On Friday I set this little puzzle…..I have come up with a way of sorting numbers into one of three groups. Here are some examples of the numbers in my three groups….

Group One: 0, 3, 6, 8, 9

Group Two: 1, 4, 7, 11, 14

Group Three: 2, 5, 10, 12, 13

In which group should I place the numbers 15, 16 and 17?

If you have not tried to solve it, have a go now. For everyone else the answer is after the break.

All of the numbers in Group 1 only have curves, all of those in Group 2 only have straight lines, and Group 3 have a mixture. So, 15 goes into Group 3, 16 into Group 3, and 17 into Group 2.

Did you solve it? Any other answers?

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 theiBookstore (UK here in the USA here). You can try 101 of the puzzles for free here.


  1. As others have mentioned, this depends too much on fonts to be a good puzzle (for example, on my computer viewed through Google Reader, 7 has a curve, and this page loaded directly gives 1 a slight curve on the top left). On the other hand, a font that used a digital-clock style for numbers would have nothing but straight lines.

  2. Friday it said: “A fun mathematical puzzle this week.”
    This is not mathematical.

    Anyway, I didn’t get it.

  3. ‘Friday it said: “A fun mathematical puzzle this week.”
    This is not mathematical.’

    It’s called misdirection.

  4. yes,yes, as Others have stated. on my computer the 7 has a curve. also on some computers the serif twiddles may have curves. not fair test. that’s why setting this puzzle in roman Numbers would be fairer.

  5. got it, using recursive non-derivative algebra in a leveraged geometric configuration,
    They all were in group 2 on my calculator.

  6. I agree with -M-. I read this was a mathematical puzzle so I decided to dismiss any other patterns.

    1. I noticed it actually, but did not comment. Richard isn’t very good at questions………
      on reflection he’s not very good at answers either.

  7. I have defined a function f such that f(15)=0,3,6,8,9… ; f(16)=1,4,7,11,14… ; f(17)=2,5,10,12,13… . Unfortunately, this comment box is too small to write f in full.

    *This*, my friend, is a mathematical answer.

  8. Blindingly obvious once it is pointed out. Embarrassing not to see it in a few seconds.
    Luckily I was still stuck on the two trains.

    1. Make sure you don’t make any unwarranted assumptions about ground and perpendiculars.

  9. i saw a really different pattern emerge. Usually when people start counting, they start at number 1 but this puzzle started on zero. Zero being the first number in the sequence, goes in group1. Then the number One, being the second number in the sequence, goes in group2. The number two is third so it goes in group3, and so on. that pattern continues until the number 8. 8 and 9 are in the first group (sequential numbers) and the numbers 12 and 13 are also sequential numbers and are in group3. After this, it’s complicated for me to explain . but there’s a bit more to it. Since we are dealing with numbers, one can think about them being group for many different reasons. Nonetheless, I do agree with previous comments, the original solution to the grouping wasn’t really mathematical.

  10. I didn’t solve it, because I’m lazy.
    But after reading the answer it made perfect sense.
    It is mathematical, if you look at the common factors in group 1 and 2 you would then be able to solve the problem. In it’s simplest form 1+2=3. JMO

  11. Assumptions!!!

    Did not make it explicit that 15,16,17 didn’t all go into the same group… Had me coming up with some crazy factorials

  12. I got it.
    What I don’t get is why the font sould be such a problem. When you learn to write numbers at school, you learn a standar way to draw them. So in my opinion even written in “corsiva”, 1, 7 and 4 are still known to be written with straigt lines.
    I understand better the messages that argue that this is not a mathematical puzzle. But couldn’t we consider that curves and straight lines are geometric matters, and therefore part of mathematics ?

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