My brother showed me the following puzzle from some type of (IQ?) quiz game on his phone:

1 + 2 = 21
2 + 3 = 36
3 + 4 = 43
4 + 5 = ??

I am quite sure this was the original question asked.

I pondered it for a while and had a few guesses, but none were accepted as "Correct."

I also tried to search on google for it in various ways, but got nowhere.

  • $\begingroup$ It seems easy but it is not. I have a bad feeling that it's some pop-culture reference idk. $\endgroup$ Oct 18, 2019 at 22:03
  • $\begingroup$ Can you share which guesses were incorrect? $\endgroup$
    – HTM
    Oct 18, 2019 at 22:17
  • $\begingroup$ Are you able to go back to the app and test out further solutions, or are you just wondering if anyone can come up with some logic that works for all terms? (If only that 6 were a 2, eh??) $\endgroup$
    – Stiv
    Oct 18, 2019 at 22:18
  • $\begingroup$ Oh.. I guess I know what this game is.. Tbh, I'm also stuck here. Idk if it's a legit logic puzzle or another trick silly question. (I dont remember but, have you tried 9?) $\endgroup$
    – athin
    Oct 18, 2019 at 22:34
  • 1
    $\begingroup$ Related xkcd, with numbers instead of words $\endgroup$
    – HTM
    Oct 19, 2019 at 5:51

3 Answers 3


The Correct Answer was



The question did not state that the above expressions were correct. The answer was simply 4+5=9

The app is Brain Out, and this is question 58, which answer is shown in this video.

However, I think this is a very poor answer. Were I the setter of the question, I would treat the + as an unknown operator. A more reasonable answer than the "correct one" would be:




Tens digit

is the larger of the two numbers on the LHS, i.e. 5

Units digit

Goes from 1 --(+2+3)(mod10)-> 6 --(+3+4)(mod10)-> 3 --(+4+5)(mod10)-> 2

  • $\begingroup$ Thanks. I did not expect such a "What kind of cheese is made backwards" kind of question. That being said, I like your "better" answer. $\endgroup$
    – Chowzen
    Oct 19, 2019 at 10:04
  • $\begingroup$ well, I sure hope it helps ;) @chowzen $\endgroup$ Oct 19, 2019 at 10:05
  • 2
    $\begingroup$ Curious: why did you edit my answer with the source and remove my statement of "I tried to find an answer online" statement instead of simply adding the source to your answer? It's good practice for an OP to state that they have made an effort to find a solution, yet you've removed that?? $\endgroup$
    – Chowzen
    Oct 19, 2019 at 10:11
  • $\begingroup$ Oh, sorry, just thought that wasnt necessary now that you have the source. sorry if you mind, you can edit it back ;) $\endgroup$ Oct 19, 2019 at 10:16

If "a+b=c" then if one of them is a prime then c=10b+a , if both are primes then c=a^2*b^2 Sorry for this..:)

  • $\begingroup$ nice try, but seems that this is not the kind of answer the OP was looking for. beware of downvotes, yet keep up your hard work and happy puzzling :) $\endgroup$ Oct 19, 2019 at 11:13
  • $\begingroup$ it was just a joke, such thing cannot be the answer.:) (should have posted as a comment) $\endgroup$
    – balazs.com
    Oct 19, 2019 at 14:13

1+2=21 (2~1) swap places 1+0=1 [21]
2+3=36 (3~2) swap places 2+4=6 [36]
3+4=43 (4~3) swap places 3+0=3 [43]

we can notice a broad pattern altering between adding 0 and 4.

4+5=? applying the same logic we should be getting this answer. (5~4) swap places 4+4=8. [58]


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