# Is there an answer?

Recently, on one of my math competitions, there was a problem that no one got, in clouding the teacher. So I was wondering if it's even answerable.

#### A number n has 4 distinct prime factors. Find the number of positive integers that divide n with remainder.

The answer is not:

N-16.

194

Note by Trevor Arashiro
4 years, 11 months ago

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## Comments

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Doesn't this depend on the multiplicity of the distinct prime factors?

Are we looking at $n = p_{1}^{a_{1}} * p_{2}^{a_{2}} * p_{3}^{a_{3}} * p_{4}^{a_{4}}$ for distinct primes $p_{k}$ with exponents $a_{k} \ge 1$?

If so, then the I think the desired number is $\prod_{k=1}^4 (a_{k} + 1)$.

- 4 years, 11 months ago

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Sorry, but I just realized I worded the question slightly wrong. Its supposed to be "with remainder". So for this case, its just $n-\prod_{k=1}^4(a_k+1)$

But that's what I was thinking as well. Unfortunately, it's supposedly not the answer

- 4 years, 11 months ago

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Weird. Is there any chance that the source of the supposed answer is incorrect? I see now how the "n - 16" and "194" answers arose; you are looking at $2*3*5*7 = 210$. This number definitely has $16$ divisors, and thus there would be $210 - 16 = 194$ positive integers less than $210$ that would divide into $210$ with non-zero remainder.

- 4 years, 11 months ago

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Do you know what the answer is supposed to be? That could offer a way to backtrack and figure out what they mean.

Staff - 4 years, 11 months ago

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Unfortunately not.

- 4 years, 11 months ago

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What kind of answer is expected? Is it multiple choice, integer answer, short algebraic answer, or long proof form?

Also, what does N-16 and 194 refer to?

Staff - 4 years, 11 months ago

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Answers that we tried but weren't excepted

- 4 years, 11 months ago

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If it is "with remainder" is the answer infinity?

- 4 years, 11 months ago

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That's a thought, but the 'standard' interpretation of "positive integer $m$ divides $n$ with remainder" assumes that $m \le n$. Since it is stated that $n$ has $4$ distinct prime factors I think that we can assume that this standard interpretation is being applied, otherwise this prime information would have been largely irrelevant.

- 4 years, 11 months ago

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I wasn't aware that I re-shared this. If I did, I certainly didn't intend to.

Gremlins?

- 4 years, 11 months ago

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Haha Actually, there have been some gremlins lurking on Brilliant the past few days. Strange things have been happening when posting/editing solutions, causing format changes, symbols appearing out of nowhere, etc.. So yes, your unintended re-share could have been the work of gremlins. :)

- 4 years, 11 months ago

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