# Solve the last equation

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.

• It seems easy but it is not. I have a bad feeling that it's some pop-culture reference idk. – Don Thousand Oct 18 '19 at 22:03
• Can you share which guesses were incorrect? – HTM Oct 18 '19 at 22:17
• 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??) – Stiv Oct 18 '19 at 22:18
• 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?) – athin Oct 18 '19 at 22:34
• Related xkcd, with numbers instead of words – HTM Oct 19 '19 at 5:51

9

Explanation

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

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:

Try

52

Explanation:

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

• 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. – Chowzen Oct 19 '19 at 10:04
• well, I sure hope it helps ;) @chowzen – Omega Krypton Oct 19 '19 at 10:05
• 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?? – Chowzen Oct 19 '19 at 10:11
• Oh, sorry, just thought that wasnt necessary now that you have the source. sorry if you mind, you can edit it back ;) – Omega Krypton Oct 19 '19 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..:)

• 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 :) – Omega Krypton Oct 19 '19 at 11:13
• it was just a joke, such thing cannot be the answer.:) (should have posted as a comment) – balazs.com Oct 19 '19 at 14:13