2017 AIME II Problems/Problem 9
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[hide]Problem
A special deck of cards contains cards, each labeled with a number from to and colored with one of seven colors. Each number-color combination appears on exactly one card. Sharon will select a set of eight cards from the deck at random. Given that she gets at least one card of each color and at least one card with each number, the probability that Sharon can discard one of her cards and have at least one card of each color and at least one card with each number is , where and are relatively prime positive integers. Find .
Solution 1
Without loss of generality, assume that the numbers on Sharon's cards are , , , , , , , and , in that order, and assume the colors are red, red, and five different arbituary colors. There are ways of assigning the two red cards to the numbers; we subtract because we cannot assign the two reds to the two 's. In order for Sharon to be able to remove at least one card and still have at least one card of each color, one of the reds have to be assigned with one of the s. The number of ways for this not to happen is , so the number of ways for it to happen is . Each of these assignments is equally likely, so the probability that Sharn can discard one of her cards and still have at least one card of each color and at least one card with each number is , and .
Solution 2
There have to be of cards sharing the same number and of them sharing same color.
pairs of cards can't be the same or else there will be card which are completely same.
WLOG the numbers are and and the colors are and Then we can get cases:
Case One: and in this case, we can discard . there are situations in this case.
Case Two: and In this case, we can't discard. There are situations in this case
So the probability is
The answer is
See Also
2017 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 8 |
Followed by Problem 10 | |
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