Numbers! no less than a maze

Number Theory Level pending

How many numbers less than 1000 exist such that they are 6 times the sum of their digits?


The answer is 1.

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1 solution

Marco Brezzi
Aug 13, 2017

Suppose n < 1000 n<1000 was a 3 3 -digits number

n = a b c n=\overline{abc}

To respect the constraint we need to have

100 a + 10 b + c = 6 ( a + b + c ) 94 a + 4 b = 5 c 100a+10b+c=6(a+b+c)\iff 94a+4b=5c

Since a a , b b and c c are digits and a a is non-zero:

94 a + 4 b 94 1 + 4 0 94 94a+4b\geq 94\cdot 1 +4\cdot 0\geq 94 and

5 c 5 9 45 5c\leq 5\cdot 9 \leq 45

Which is not possible, so n n has less than 3 3 digits

Suppose n = a b n=\overline{ab}

10 a + b = 6 a + 6 b 4 a = 5 b a = 5 , b = 4 \Longrightarrow 10a+b=6a+6b\iff 4a=5b \iff a=5,b=4

n n cannot have only one digit, since that would imply

a = 6 a a = 0 a=6a \iff a=0

So there is only 1 \boxed{1} such n n

Note: \textbf{Note:}

You should include that we are talking about positive integers, otherwise 0 0 would indeed be a solution

From the linear Diophantine equation 94 a + 4 b = 5 c 94a + 4b = 5c , we can also use Bezout's identity to lessen the work, observing that gcd ( 94 , 4 ) = 2 \gcd(94, 4) = 2 which does not divide 5 5 .

Zach Abueg - 3 years, 10 months ago

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Uhm, not really. On the R H S RHS we have 5 c 5c , not 5 5 . 2 2 doesn't divide 5 5 but it can divide c c

Marco Brezzi - 3 years, 10 months ago

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My mistake, thank you.

Zach Abueg - 3 years, 10 months ago

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