1988 AIME Problem 15

Attempt Problem 15 of the 1988 AIME below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1988 AIME solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

15.

In an office at various times during the day, the boss gives the secretary a letter to type, each time putting the letter on top of the pile in the secretary’s in-box. When there is time, the secretary takes the top letter off the pile and types it. There are nine letters to be typed during the day, and the boss delivers them in the order 1,1, 2,2, 3,3, 4,4, 5,5, 6,6, 7,7, 8,8, 9.9.

While leaving for lunch, the secretary tells a colleague that letter 88 has already been typed, but says nothing else about the morning’s typing. The colleague wonders which of the nine letters remain to be typed after lunch and in what order they will be typed. Based upon the above information, how many such after-lunch typing orders are possible? (That there are no letters left to be typed is one of the possibilities.)

Answer: 704
Concepts:arrangements with restrictionssubsetsmultiplication principle
Difficulty rating: 2520
Small Hint:

After 88 has been typed, any remaining letters among 1,,71,\ldots,7 must later be typed in decreasing order

Big Hint:

Separate the cases according to whether 99 was typed before lunch or remains to be inserted into the later order

Solution:

Any subset of 1,,71,\ldots,7 can remain in the stack after 88 has been typed, and those remaining letters must later be typed in decreasing order. If 99 was already typed, choosing that subset gives 27=1282^7=128 possible orders.

If 99 remains, choose kk of the seven smaller letters and insert 99 into any of the k+1k+1 positions in their decreasing order. This gives k=07(7k)(k+1)=726+27=576.\begin{aligned}\sum_{k=0}^7\binom7k(k+1)&=7\cdot2^6+2^7\\&=576.\end{aligned} Every such order can be realized by suitable morning choices, so the total is 128+576=704.128+576=704.

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