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jmc

number theory senior

Problem

For how many integer values of between 1 and 1000 inclusive does the decimal representation of terminate?
Solution
The decimal representation of a simplified fraction terminates if and only if the denominator is divisible by no primes other than 2 and 5. The prime factorization of is . For the fraction to simplify to having only the primes and in the denominator, there must be a factor of in the numerator. There are multiples of between and , so there are integers values for .
Final answer
90