2020 AMC 10B Problems/Problem 17
- The following problem is from both the 2020 AMC 10B #17 and 2020 AMC 12B #15, so both problems redirect to this page.
Contents
Problem
There are people standing equally spaced around a circle. Each person knows exactly
of the other
people: the
people standing next to her or him, as well as the person directly across the circle. How many ways are there for the
people to split up into
pairs so that the members of each pair know each other?
Solution
Let us use casework on the number of diagonals.
Case 1: diagonals
There are
ways: either
pairs with
,
pairs with
, and so on or
pairs with
,
pairs with
, etc.
Case 2: diagonal
There are
possible diagonals to draw (everyone else pairs with the person next to them.
Note that there cannot be 2 diagonals.
Case 3: diagonals
Note that there cannot be a case with 4 diagonals because then there would have to be 5 diagonals for the two remaining people, thus a contradiction.
Case 4: diagonals
There is
way to do this.
Thus, in total there are possible ways.
Video Solution
https://youtu.be/3BvJeZU3T-M (for AMC 10) https://youtu.be/0xgTR3UEqbQ (for AMC 12)
~IceMatrix
See Also
2020 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 16 |
Followed by Problem 18 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
2020 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 14 |
Followed by Problem 16 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
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