Today I went to pick up my daughter from school before the start of the winter holidays. I found her chatting with her teacher and they presented me with a little math puzzle. It was on a card from a set of cards with little math puzzles that the teacher had bought somewhere. Most other puzzles were simple, sensible math puzzles, and they had solved them all, except this one. And it left me stumped too.

The puzzle was simple: find the missing number:

3 4 11
5 16 29
6 25 ??

The design on the card had a strong triangle theme, with the numbers in each triplet being arranged in a triangular pattern, and all three triplets were printed on the vertices on a triangle as well. However that could also be just an aesthetic choice.

I didn't inspect the pack too closely, but the puzzles seemed to be meant for children around 10-12 years of age, around grades 4-6. So nothing too fancy - no trigonometry, geometry, complex numbers or whatever. Really simple, basic stuff, with just a smidgeon more of logic and trickiness added, since they were meant to be "hard" problems (for the age group).

The pack also had an answer card, and the correct number in the spot is supposed to be


However it did not include any explanations why so. Just an answer. I've been thinking about this all day long, and I cannot figure it out. Best I've come up with is:

a + b + 2*sqrt(b) = c

However that seems way too convoluted and probably is just a coincidence. I've also noted that

The second number is always a perfect square, and the last number is a prime

But whether that has any significance - probably not.

So - any ideas? :)

  • $\begingroup$ Note also that b = (a-1)^2, and c = a(a+1)-1. b seems extraneous, but you could probably shoehorn a lot of different patterns in here. $\endgroup$
    – Skynet_0
    Dec 21, 2019 at 0:49
  • $\begingroup$ @Skynet_0 - Yes, that's the thing I usually don't like about those "missing number" puzzles. With enough pareidolia, there's no end to different answers. $\endgroup$
    – Vilx-
    Dec 21, 2019 at 1:20

1 Answer 1


A possibility:

Add the square of the first number to the square root of the second number.

Mathematically: $a^2 + \sqrt{b}=c$

A student knowing about squares and square roots might reach this answer.

  • $\begingroup$ Hmm, I like this one! But would it be of appropriate level? I'll check. $\endgroup$
    – Vilx-
    Dec 21, 2019 at 15:12
  • $\begingroup$ I asked, and I guess that such operations as squaring and taking a square root were not part of the cards. Best they had was percentages. However... this was the hardest card of them all, and the numbers are small enough for this to be grokked intuitively, I guess... I'll accept your answer, until a better one comes along. $\endgroup$
    – Vilx-
    Dec 21, 2019 at 15:20
  • $\begingroup$ Hah! We found the same cards in the store and bought them today. Reading of fine print revealed that there's a downloadable Android app with the full video answers. Long story short - you're right! This is exactly what it means. Also turns out there's a bunch more of these in the pack and that my daughter and the teacher hadn't actually gotten to them yet. :P Thank you for your time! :) $\endgroup$
    – Vilx-
    Dec 22, 2019 at 18:15

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