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PROJECT EULER · #0073

Counting Fractions in a Range

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Consider the fraction, nd, where n and d are positive integers. If n<d and HCF(n,d)=1, it is called a reduced proper fraction.

If we list the set of reduced proper fractions for d8 in ascending order of size, we get: 18,17,16,15,14,27,13,38,25,37,12,47,35,58,23,57,34,45,56,67,78

It can be seen that there are 3 fractions between 13 and 12.

How many fractions lie between 13 and 12 in the sorted set of reduced proper fractions for d12000?

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