Sum The Digits
Logic
Level
2
If the sum of the digits of a number is equal to the sum of the digits of twice the number, the number must be a multiple of...
11
5
7
2
3
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When you multiply 9 by 2, you get 18. Notice that the sum of the digits in both numbers are the same.
As you keep trying different numbers, you start to notice that only multiples of 9 works.
All multiples of 9 are multiples of 3, so the answer is 3.