Problem:
A positive integer is called "flippant" if does not end in (when written in decimal notation) and, moreover, and the number obtained by reversing the digits of are both divisible by . How many flippant integers are there between and ?
Solution
Solution:
We use the notation "" to mean "divides."
There is only one flippant 2-digit number, namely . Indeed, if is flippant (where are integers -), then and . Thus,
so that , and similarly , so we'd better have .
There are flippant 3-digit numbers. First consider the palindromic ones (ones where the hundreds and units digits are the same): , and .
Now consider the general case: suppose is flippant, where are integers -. Then and , so , and so . In order for this not to result in a palindromic integer, we must have and, moreover, both and must be palindromic flippant integers. Consulting our list above, we find more flippant integers: , and .
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