2001 AMC 12 Problem 16

Below is the professionally curated solution for Problem 16 of the 2001 AMC 12, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2001 AMC 12 solutions, or check the answer key.

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Concepts:arrangements with restrictionspermutations

Difficulty rating: 1600

16.

A spider has one sock and one shoe for each of its eight legs. In how many different orders can the spider put on its socks and shoes, assuming that, on each leg, the sock must be put on before the shoe?

8!8!

288!2^8\, 8!

(8!)2(8!)^2

16!28\dfrac{16!}{2^8}

16!16!

Solution:

Think of the 1616 items (88 socks and 88 shoes) arranged in some order: there are 16!16! arrangements.

For each leg, the sock comes before the shoe in exactly half of all arrangements. Imposing this on all eight legs independently divides by 28,2^8, giving 16!28. \dfrac{16!}{2^8}.

Thus, the correct answer is D.

Problem 16 in Other Years