AMC 10 Step by Step
2022 AMC 10AProblem 22P21-25~7 minPrint

Suppose that 1313 cards numbered 1,2,3,,131, 2, 3, \ldots, 13 are arranged in a row. The task is to pick them up in numerically increasing order, working repeatedly from left to right. In the example below, cards 1,2,31, 2, 3 are picked up on the first pass, 44 and 55 on the second pass, 66 on the third pass, 7,8,9,107, 8, 9, 10 on the fourth pass, and 11,12,1311, 12, 13 on the fifth pass. For how many of the 13!13! possible orderings of the cards will the 1313 cards be picked up in exactly two passes?

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Problem © Mathematical Association of America (MAA), American Mathematics Competitions. Reproduced for non-commercial educational use. Solution and commentary are original to this site.