Math logic puzzler for fun

[ERD50]

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.

I can't agree with that at all. As Philliefan33 explains, once you change the meaning of the math symbols, it all falls apart. If a word/symbol means whatever I want it to mean (Alice in Wonderland I think?), then the answer can be anything anyone chooses.

I like "42", it is the answer to everything.

As a puzzle, I'd go with 34, but I think the other derivations for 45 are as, or more, valid.

-ERD50

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.

 

[chilkoot]

Another vote for: 34, but other answers (like 45) would work as well.

 

[HFWR]

Usually, this type of puzzle is designed to see if you can extrapolate a pattern. Skipping steps, in this case 4 + 7, is a way to determine if you found the pattern. I seem to recall questions of this design on standardized tests, such as the SAT.

 

[imoldernu]

72 without overthinking

 

[RunningBum]

I'm going to go ahead and give the answer in the next post. The thread is on page 2 now, and if someone really wants time to try to figure out what I'm looking for, don't read past this post.

 

[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.

The answer to 5 + 8, is, very simply, 13.

I didn't redefine the meaning of the standard '+' operation. IMO that would be wrong.

But...but...but, what about the 2nd and 3rd answers?? Well, they are wrong! <ducking to avoid thrown shoes and rotten fruit>

The lesson is to not necessarily follow the mistakes of others, looking for patterns to justify an incorrect assumption. '+' means exactly what it's defined as. Had some other undefined symbol been used instead, one could look for a more complex formula.

We have a saying in trail running, "Don't follow the idiot in front of you", meaning, keep your eyes on the trail markers, and follow those, rather than blindly following other runners who may go off trail and get lost. IMO it applies all over in life. Certainly it can in personal finance and investing. I can remember a number of cases in my software developer life where I was stuck on a difficult problem and finally found and fixed it by reexamining the assumptions I'd been making.

Like some others, when I first saw this puzzle I got a little angered at someone trying to make up new rules for the '+' operation. So I came up with my own answer, and the more I thought about it, the more sense it made to me, and the more I liked the lesson.

On a nerdish technicality, I was not exactly correct that this is the only answer. "D" would also be correct, if you are using the hexadecimal numbering system (and so on with other even less standard systems).

Thanks for playing, and while it was deceptive, my intent was not to upset anyone, and certainly not to make anyone feel dumb. It was, however, to suggest that people might step back and look at things a bit differently, and not just accept assumptions as a given.

 

[Ready]

29 works.

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

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

You could also multiply 8x4 assuming the number being multiplied is just increasing by 1 unit for each successive line.

1 + (4x1) = 5
2 + (5x2) = 12
3 + (6x3) = 21
5 + (8x4) = 37


But I'm sure there is more than one way to interpret this challenge, so I don't know that there is only one right answer.

 

[ERD50]

The answer to 5 + 8, is, very simply ....

OK, I retract my previous comment and accept your answer. You are correct, just because some other statements are wrong, it does not lead to anything different to solve for X. Being presented as a "puzzle" leads one to look for patterns, but that doesn't make it right!

-ERD50

 

[RunningBum]

OK, I retract my previous comment and accept your answer. You are correct, just because some other statements are wrong, it does not lead to anything different to solve for X. Being presented as a "puzzle" leads one to look for patterns, but that doesn't make it right!

-ERD50

Yeah, I shouldn't have called this a puzzle, as I was trying to make this as straightforward as possible to not further mislead.

 

[GrayHare]

Now the standard reply to "Do I have enough?" questions here can be, "Can't say, some of your addition may be wrong."

 

[FIREmenow]

The answer to 5 + 8, is, very simply, 13.

I didn't redefine the meaning of the standard '+' operation. IMO that would be wrong.

But...but...but, what about the 2nd and 3rd answers?? Well, they are wrong! <ducking to avoid thrown shoes and rotten fruit>

The lesson is to not necessarily follow the mistakes of others, looking for patterns to justify an incorrect assumption. '+' means exactly what it's defined as. Had some other undefined symbol been used instead, one could look for a more complex formula.

We have a saying in trail running, "Don't follow the idiot in front of you", meaning, keep your eyes on the trail markers, and follow those, rather than blindly following other runners who may go off trail and get lost. IMO it applies all over in life. Certainly it can in personal finance and investing. I can remember a number of cases in my software developer life where I was stuck on a difficult problem and finally found and fixed it by reexamining the assumptions I'd been making.

Like some others, when I first saw this puzzle I got a little angered at someone trying to make up new rules for the '+' operation. So I came up with my own answer, and the more I thought about it, the more sense it made to me, and the more I liked the lesson.

On a nerdish technicality, I was not exactly correct that this is the only answer. "D" would also be correct, if you are using the hexadecimal numbering system (and so on with other even less standard systems).

Thanks for playing, and while it was deceptive, my intent was not to upset anyone, and certainly not to make anyone feel dumb. It was, however, to suggest that people might step back and look at things a bit differently, and not just accept assumptions as a given.

Brilliant.

 

[ERD50]

Yeah, I shouldn't have called this a puzzle, as I was trying to make this as straightforward as possible to not further mislead.

It's OK, I may have been misunderstood when I said "Being presented as a "puzzle" leads one to look for patterns, but that doesn't make it right!"

I meant that looking for patterns was not right on my part. I wasn't criticizing presenting it as a puzzle. After all, if it was all so obvious, it wouldn't be very interesting/entertaining would it?

And as you said, it really shows that we shouldn't let extraneous info warp our thinking. When I am asked to help someone with a tech problem, after they describe the problem, I often go into 'interview mode', starting at step one, and going step by step. They get aggravated "I told you the problem I'm having!", but their description often leads me astray, because I didn't have the full context, or what they did before/after, and the only way I can get all the meaningful info I need is step by step. And ignore the stuff that doesn't matter.

Good thread! You got me!

-ERD50

 

[Car-Guy]

After reading this thread, my answer is, I can only hope none of you math majors here had anything to do with writing the computational code for FIRECal.

 

[Just_Steve]

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

Forth post in and Zinger shoots, Zinger scores.

 

[Sunset]

Yeah, I shouldn't have called this a puzzle, as I was trying to make this as straightforward as possible to not further mislead.

Calling it a puzzle of course made the 2 possible algorithms that fit stand out, rather than the regular answer.

 

[lucky penny]

45

 

[donheff]

34

 

[donheff]

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 !!

+1 Really POd me when I guessed they were trying to trick you and gave a sophisticated (correct) answer and got marked wrong because they wanted the simple but arguably wrong answer.

 

[Which Roger]

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.

Language is imprecise and therefore ambiguous, hence there are several ways to view this and therefore many acceptable answers.

From the perspective of predicate logic, this can be read as
"if ((1+4=5) and (2+5=12) and (3+6=21)) then (5+8=X))". Here, any value of X makes this a true statement. "If A then B" is logically equivalent to "(NOT A) OR B", and since the antecedent is false, the statement is true regardless of what value we choose for X.

Or, as many of the answers have suggested, the meaning of the + operator can be redefined, like in object-oriented programming, where the meaning of the operator depends on the class of the message recipient, i.e., the first operand.

Or, as the OP suggested, it can be read as a sequence of statements, some true and some false, and the correct answer is 13.

 

[Car-Guy]

+1 Really POd me when I guessed they were trying to trick you and gave a sophisticated (correct) answer and got marked wrong because they wanted the simple but arguably wrong answer.

+2, can't trust anyone these days.

 

[OldShooter]

Tough crowd.

 

[RunningBum]

Language is imprecise and therefore ambiguous, hence there are several ways to view this and therefore many acceptable answers.

From the perspective of predicate logic, this can be read as
"if ((1+4=5) and (2+5=12) and (3+6=21)) then (5+8=X))". Here, any value of X makes this a true statement. "If A then B" is logically equivalent to "(NOT A) OR B", and since the antecedent is false, the statement is true regardless of what value we choose for X.

Or, as many of the answers have suggested, the meaning of the + operator can be redefined, like in object-oriented programming, where the meaning of the operator depends on the class of the message recipient, i.e., the first operand.

Or, as the OP suggested, it can be read as a sequence of statements, some true and some false, and the correct answer is 13.

Good analysis. I have no argument against any of this.

 

[VT2FIRE]

45. Following the pattern, the sum of 21+4+7=32, then the sum of 32+5+8=45. Sorry to spill the beans but didn't want you to think I was repeating prior posters.

 

[Sirka]

32+5+8=45

 

[MichaelB]

Language is imprecise and therefore ambiguous, hence there are several ways to view this and therefore many acceptable answers.

From the perspective of predicate logic, this can be read as
"if ((1+4=5) and (2+5=12) and (3+6=21)) then (5+8=X))". Here, any value of X makes this a true statement. "If A then B" is logically equivalent to "(NOT A) OR B", and since the antecedent is false, the statement is true regardless of what value we choose for X.

Or, as many of the answers have suggested, the meaning of the + operator can be redefined, like in object-oriented programming, where the meaning of the operator depends on the class of the message recipient, i.e., the first operand.

Or, as the OP suggested, it can be read as a sequence of statements, some true and some false, and the correct answer is 13.

Good analysis. I have no argument against any of this.

What would Occam's razor suggest as the better answer?