Math logic puzzler for fun

[RunningBum]

Solve this:

1 + 4 = 5
2 + 5 = 12
3 + 6 = 21
5 + 8 = ?

What is the last answer?

I'll post the answer sometime this evening (eastern time), to give people a chance to answer.

 

[SteveNU]

45

 

[DFA]

34

 

[zinger1457]

Looks like a trick question, I'll go with 13.

 

[Wheel]

45

 

[dasranger]

45

 

[bradaz2488]

34

 

[FIREmenow]

45

 

[Philliefan33]

34

I should move this to the "pet peeves" post. Puzzles like this are nice little brain teasers, but the syntax is totally nonsensical and are like nails on a chalkboard to me. The + and = symbols have very specific meaning. One should not just throw around terms such as " 2+5=12".

It is bad math grammar.

 

[John Galt III]

Probably more than one answer. The one that stands out to me is 34.

 

[scrabbler1]

I like 45, too.

 

[bw5972]

45

“a + b” then answer is “a + (a x b)” in each case.

 

[traineeinvestor]

Any of 13, 34 and 45 can be supported with logic which I suppose is the whole point of the exercise.

 

[bradaz2488]

Any of 13, 34 and 45 can be supported with logic which I suppose is the whole point of the exercise.

Yep, I'm waiting to see what answer tells you about "you"

 

[FIREmenow]

34

I should move this to the "pet peeves" post. Puzzles like this are nice little brain teasers, but the syntax is totally nonsensical and are like nails on a chalkboard to me. The + and = symbols have very specific meaning. One should not just throw around terms such as " 2+5=12".

It is bad math grammar.

 

[ChiliPepr]

on first look I can see at least 3 answers that could work.... 34, 37 or 45, but I could probably find more that would work also.

The problem with questions like this is it is a more sophisticated version of if a+b=17, what is the value of A?

 

[ChiliPepr]

34

I should move this to the "pet peeves" post. Puzzles like this are nice little brain teasers, but the syntax is totally nonsensical and are like nails on a chalkboard to me. The + and = symbols have very specific meaning. One should not just throw around terms such as " 2+5=12".

It is bad math grammar.

+1

 

[Lawrencewendall]

34

 

[zinger1457]

34
It is bad math grammar.

Agree, math sequence puzzles usually have commas between the numbers, that's why I picked 13.

 

[John Galt III]

Reminds me of those aptitude tests that you had to take in high school. You saw many different possible answers to one question, and you wondered if the folks who wrote the questions even realized that !!

 

[OldShooter]

29 works.

Edit: Oops, I didn't read carefully enough. Try:

1 + (4x1) = 5
2 + (5x2) = 12
3 + (6x3) = 21
5 + (8x5) = 45

 

[GrayHare]

Any good coder is all over this sort of puzzle since debugging incorrect program output involves looking for patterns.

 

[Tom52]

45 seems to fit the sequence.

 

[RunningBum]

I'll give 2 hints.


As far as I'm concerned, there is one and only one correct answer. I'll explain when I reveal it.


Don't overthink the math.

 

[Totoro]

1 + (4x1) = 5
2 + (5x2) = 12
3 + (6x3) = 21
5 + (8x5) = 45

Or carry-over math:
1+4=5
[5+] 2+5= 12
[12+] 3+6=21
[21+] 5+8=34

Fun fact: you can google "3 + 6 = 21" for spoilers

Anyway, fully agree you can't toss around symbols like this without reframing. Introducing a 'running total' concept removes the convention that every line stands by itself with this notation.

Which is a less gross violation than if '45' would be defined as a possible solution, that's just changing the equation altogether.

It's like the following puzzle:
1 + 4 = 7
2 + 5 = 7
3 + 6 = 8
5 + 8 = ?

Which involves redefining the '+' symbol somewhat. Or a puzzle where you need to change a convention most people don't even realize is there.
1 + 4 = 10
2 + 5 = 12
3 + 6 = 14
5 + 8 = ?