Problem of the Week 1240
Nondivisibility by 11
The number 545 has the curious property that upon replacing the digit in any single position by an arbitrary digit (from 0 to 9; it can be a leading 0 or just the same digit), the result is not divisible by 11.
Is there a positive integer with this property having an even number of digits?
Source: Sid Hollander, who found the ambiguity.