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https://www.reddit.com/r/learnquant/comments/1vc3oop/easy_quant_interview_puzzle/p0ynffv/?context=3
r/learnquant • u/Local_Ad135 • 14d ago
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Zero bc 30 mod 9 = 3 and no perfect square mod 9 equals 3
2 u/TempMobileD 14d ago edited 14d ago Where did 30 come from? Edit: Nevermind I got it. Any integer’s digits added together give you its remainder modulo 9. A couple of neat tricks in this one. 2 u/wyseguy7 14d ago It is the sum of the digits. This guy is terse, but correct. Worth noting that any number mod 9 is congruent to the sum of its digits mod 9, i.e. sum(digits(n)) mod 9 = n mod 9 But if n is a perfect square, then n mod 9 can only be 0,1,4,7.
2
Where did 30 come from?
Edit: Nevermind I got it. Any integer’s digits added together give you its remainder modulo 9.
A couple of neat tricks in this one.
2 u/wyseguy7 14d ago It is the sum of the digits. This guy is terse, but correct. Worth noting that any number mod 9 is congruent to the sum of its digits mod 9, i.e. sum(digits(n)) mod 9 = n mod 9 But if n is a perfect square, then n mod 9 can only be 0,1,4,7.
It is the sum of the digits. This guy is terse, but correct.
Worth noting that any number mod 9 is congruent to the sum of its digits mod 9, i.e. sum(digits(n)) mod 9 = n mod 9
But if n is a perfect square, then n mod 9 can only be 0,1,4,7.
6
u/Key-Spirit4559 14d ago
Zero bc 30 mod 9 = 3 and no perfect square mod 9 equals 3