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Printjmc
number theory senior
Problem
If is the probability that the reciprocal of a randomly selected positive odd integer less than 2010 gives a terminating decimal, with and being relatively prime positive integers, what is ?
Solution
There are 2009 positive integers less than 2010, of which 1005 are odd. If is equal to a terminating decimal, then can only be divisible by 2 and 5. However, since we have the added restriction that is odd, must be a power of 5. There are five powers of 5 less than 2010. Note that . Since there are five odd integers that satisfy our desired condition, the desired probability is . This is in simplest terms, so our answer is .
Final answer
202