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smc

algebra senior

Problem

Let be the set of positive integers that have no prime factors other than , , or . The infinite sum of the reciprocals of the elements of can be expressed as , where and are relatively prime positive integers. What is ?
(A)
(B)
(C)
(D)
Solution
Note that the fractions of the form where and are nonnegative integers, span all terms of the infinite sum. Therefore, the infinite sum becomes by a product of geometric series, from which the answer is
Final answer
C