2021 AMC 10B Problems/Problem 22
Contents
Problem
Ang, Ben, and Jasmin each have blocks, colored red, blue, yellow, white, and green; and there are
empty boxes. Each of the people randomly and independently of the other two people places one of their blocks into each box. The probability that at least one box receives
blocks all of the same color is
, where
and
are relatively prime positive integers. What is
Solution
Let our denominator be , so we consider all possible distributions.
We use PIE (Principle of Inclusion and Exclusion) to count the successful ones.
When we have at box with all
balls the same color in that box, there are
ways for the distributions to occur (
for selecting one of the five boxes for a uniform color,
for choosing the color for that box,
for each of the three people to place their remaining items).
However, we overcounted those distributions where two boxes had uniform color, and there are ways for the distributions to occur (
for selecting two of the five boxes for a uniform color,
for choosing the color for those boxes,
for each of the three people to place their remaining items).
Again, we need to re-add back in the distributions with three boxes of uniform color... and so on so forth.
Our success by PIE is
yielding an answer of
.
Solution 2
As In Solution 1, the probability is
Dividing by
, we get
Dividing by
, we get
Dividing by
, we get
.
Solution 3
Use complementary counting.
Denote as the total number of ways to put
colors to
boxes by 3 people out of which
ways are such that no box has uniform color. Notice
. From this setup we see the question is asking for
. To find
we want to exclude the cases of a) one box of the same color, b) 2 boxes of the same color, c) 3 boxes of same color, d) 4 boxes of the same color, and e) 5 boxes of the same color. Cases d) and e) coincide. From this, we have
In case b), there are ways to choose 2 boxes that have the same color,
ways to choose the two colors, 2! ways to permute the 2 chosen colors, and
ways so that the remaining 3 boxes don’t have the same color. The same goes for cases a) and c). In case e), the total number of ways to permute 5 colors is
. Now, we just need to calculate
,
and
.
We have , since we subtract the number of cases where the boxes contain uniform colors, which is 2.
In the same way, - again, we must subtract the number of ways at least 1 box has uniform color. There are
ways if 3 boxes each contains uniform color. Two boxes each contains uniform color is the same as previous. If one box has the same color, there are
ways to pick a box, and
ways to pick a color for that box, 1! ways to permute the chosen color, and
ways for the remaining 2 boxes to have non-uniform colors. Similarly,
Thus,
Thus, the probability is and the answer is
-angelinasheeen
Video Solution by OmegaLearn (Principal of Inclusion Exclusion)
~ pi_is_3.14
Video Solution by Interstigation
~ Briefly went over Principal of Inclusion Exclusion using Venn Diagram
2021 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 21 |
Followed by Problem 23 | |
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