2016 AMC 12B Problems/Problem 20
Contents
Problem
A set of teams held a round-robin tournament in which every team played every other team exactly once. Every team won games and lost
games; there were no ties. How many sets of three teams
were there in which
beat
,
beat
, and
beat
Solution 1
We use complementary counting. First, because each team played other teams, there are
teams total. All sets that do not have
beat
,
beat
, and
beat
have one team that beats both the other teams. Thus we must count the number of sets of three teams such that one team beats the two other teams and subtract that number from the total number of ways to choose three teams.
There are ways to choose the team that beat the two other teams, and
to choose two teams that the first team both beat. This is
sets. There are
sets of three teams total. Subtracting, we obtain
, thus
is our answer.
Solution 2
As above, note that there are 21 teams, and call them A, B, C, ... T, U. WLOG, assume that A beat teams B-L and lost to teams M-U. We will count the number of sets satisfying the conditions of the problem that include A, then multiply by 21 (for each other team) and divide by 3 (since every set will be counted by each of the 3 teams that are apart of that set). To do this, let X and Y
. Since a total of
losses total were suffered by teams in Y and
losses were suffered by teams in Y from teams in Y, we have
losses suffered by teams in Y from teams in X. Hence, for each of these
losses, there is exactly one set of three teams that includes A that satisfies the problem conditions. Thus, the answer is
.
Solution 3 (very very risky—only try if you're running out of time)
Note that there are teams total and
ways to pick
The possible arrangements are one team beats the other two or they each win/lose equally (we want the second case). Approximately
of all the arrangements satisfy the second case, and
which is by far the closest to
See Also
2016 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 19 |
Followed by Problem 21 |
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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