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counting and probability senior

Problem

How many ways are there to split the integers through into pairs such that in each pair, the greater number is at least times the lesser number?
(A)
(B)
(C)
(D)
(E)
Solution
Clearly, the integers from through must be in different pairs, and must pair with Note that can pair with either or From here, we consider casework: If pairs with then can pair with one of After that, each of does not have any restrictions. This case produces ways. If pairs with then can pair with one of After that, each of does not have any restrictions. This case produces ways. Together, the answer is
Final answer
E