How many numbers less than 1000 exist such that they are 6 times the sum of their digits?
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Suppose n < 1 0 0 0 was a 3 -digits number
n = a b c
To respect the constraint we need to have
1 0 0 a + 1 0 b + c = 6 ( a + b + c ) ⟺ 9 4 a + 4 b = 5 c
Since a , b and c are digits and a is non-zero:
9 4 a + 4 b ≥ 9 4 ⋅ 1 + 4 ⋅ 0 ≥ 9 4 and
5 c ≤ 5 ⋅ 9 ≤ 4 5
Which is not possible, so n has less than 3 digits
Suppose n = a b
⟹ 1 0 a + b = 6 a + 6 b ⟺ 4 a = 5 b ⟺ a = 5 , b = 4
n cannot have only one digit, since that would imply
a = 6 a ⟺ a = 0
So there is only 1 such n
Note:
You should include that we are talking about positive integers, otherwise 0 would indeed be a solution