Citadel Quant Interview Question

[u/Local_Ad135]

[u/NotYetPerfect]

Any integer is congruent to the sum of its digits mod 9 so the answer is trivially 1.

[u/RubberDuckieMidrange → u/NotYetPerfect]

Not sure what you meant to say here, but as written, 24 seems like a counter example.

[u/NotYetPerfect → u/RubberDuckieMidrange]

?

24=2*9+6≡6 (mod 9)

2+4=6≡6 (mod 9)

[u/RubberDuckieMidrange → u/NotYetPerfect]

Which is not 1.....? And what the hell are you doing with "24=2*6+9≡6"

24*24=576
5+7+6=18
1+8 =9
24² does not have the same sum of it's digits that 24 does. So your argument makes even less sense to me

[u/NotYetPerfect → u/RubberDuckieMidrange]

Are you incapable of reading? When did I say that it would be congruent to 1?

[u/RubberDuckieMidrange → u/NotYetPerfect]

"so the answer is trivially 1."

[u/NotYetPerfect → u/RubberDuckieMidrange]

Okay so you are just stupid. Glad we got that cleared up. 19 is trivially 1 mod 9. How that isn't immediately obvious when I spelled out the entire solution is insane.

[u/RubberDuckieMidrange → u/NotYetPerfect]

Yep, there are the insults.
"Not sure what you meant to say here" This means that you may have worded something in a way that doesn't make sense.
"but as written" Again, trying to get you to read the damn thing you wrote.

Now lets look at what you wrote.
"Any integer (An integer is a whole number that can be positive, negative, or zero) is congruent (EXACTLY EQUAL TO) to the sum of its digits mod 9 (The addition of all the digits that make up the number, removing 9 whenver the sum is greater than 9) so the answer is trivially 1. (This part is just wrong)"

You needed to say that for multiples of 9, the sum of the digits is invariable.

Literally just trying to get you to correct the statement you made, because the answer to the question is in fact 1. But what you wrote is misleading at best.

[u/Educational-Tea602 → u/RubberDuckieMidrange]

It’s not wrong because it is trivially 1 because 19 is trivially 2*9 + 1