2017 AIME II Problems/Problem 9
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 arbitrary 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
.
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
Solution 3
There are ways to choose a set of 7 cards that have all the numbers from 1-7 and all 7 colors. There are then
cards remaining. Thus, there are
desired sets.
Now, the next thing to find is the number of ways to choose 8 cards where there is not a set of 7 such cards. In this case, one color must have 2 cards and one number must have 2 cards, and they can't be the same number/color card. The number of ways to pick this is equal to a multiplication of ways to pick 2 numbers,
colors to assign them to,
ways to pick 2 nonchosen colors,
ways to pick a number to assign them to, and
ways to assign the rest.
Thus, the answer is . Dividing out
yields
which is equal to
which is equal to
which is equal to
giving a final answer of
.
See Also
2017 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 8 |
Followed by Problem 10 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
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