11 (eleven)
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The digit appears eleven time from 1 to 100.
Let's solve it a step at a time. For the first digit, how many choices do you have? 9 You can choose 1..9 but not 0, so that's nine choices for the most significant digit. For the second digit, how many choices do you have? 10 It can be 0..9. For the third digit, you also have 10 choices. Choosing one digit doesn't limit your choices for other digits and mirrored numbers (e.g. 123 and 321) are different, so all choices make a unique number. So the total is the product of our three choices: 9x10x10
This is not possible, since there are only five single digit odd numbers, which are 1, 3, 5, 7 and 9.
In such cases, you should compare one digit at a time, from left to right, until you find a digit that is different in the two numbers. That is, compare the first digit (after the decimal period) with the first digit, the second digit with the second digit, etc.
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