Difference between revisions of "2015 AMC 12A Problems/Problem 13"
Xhypotenuse (talk | contribs) |
Tecilis459 (talk | contribs) m (Fix li tag) |
||
| (3 intermediate revisions by one other user not shown) | |||
| Line 10: | Line 10: | ||
==Solution 1== | ==Solution 1== | ||
| − | We can eliminate answer choices <math>\textbf{(A)}</math> and <math>\textbf{(B)}</math> because there are an even number of scores, so if one is false, the other must be false too. Answer choice <math>\textbf{(C)}</math> must be true since every team plays every other team, so it is impossible for two teams to lose every game. Answer choice <math>\textbf{(D)}</math> must be true since each game gives out a total of two points, and there are <math>\binom{12}{2} = 66</math> games, for a total of <math>132</math> points. Answer choice <math>\boxed{\textbf{(E)}}</math> is false. If everyone draws each of their 11 games, then every team will tie for first place with 11 points each. | + | We can eliminate answer choices <math>\textbf{(A)}</math> and <math>\textbf{(B)}</math> because there are an even number of scores, so if one is false, the other must be false too. Answer choice <math>\textbf{(C)}</math> must be true since every team plays every other team, so it is impossible for two teams to lose every game. Answer choice <math>\textbf{(D)}</math> must be true since each game gives out a total of two points, and there are <math>\binom{12}{2} = 66</math> games, for a total of <math>132</math> points. Answer choice <math>\boxed{\textbf{(E)}}</math> is false. If everyone draws each of their 11 games, then every team will tie for first place with 11 points each. |
| + | |||
Quick question, why does the answer key say 3? | Quick question, why does the answer key say 3? | ||
==Solution 2== | ==Solution 2== | ||
We will proceed by process of elimination: | We will proceed by process of elimination: | ||
| + | |||
<math>\textbf{(A)}</math>: We know that this must be true, since any tied results in a 1 point (which is odd) for both teams. Hence, there must be 0 or a positive even number of odd scores. | <math>\textbf{(A)}</math>: We know that this must be true, since any tied results in a 1 point (which is odd) for both teams. Hence, there must be 0 or a positive even number of odd scores. | ||
| + | |||
<math>\textbf{(B)}</math>: This is true too, because each non-tie generates 2 points for the winner, and 0 points for the loser, which are both even scores. Hence, there must be 0 or a positive even number of even scores as well. | <math>\textbf{(B)}</math>: This is true too, because each non-tie generates 2 points for the winner, and 0 points for the loser, which are both even scores. Hence, there must be 0 or a positive even number of even scores as well. | ||
| + | |||
<math>\textbf{(C)}</math>: This must be true since every team plays every other team, so it is impossible for two teams to lose every game. | <math>\textbf{(C)}</math>: This must be true since every team plays every other team, so it is impossible for two teams to lose every game. | ||
| − | <math>\textbf{(D)}</math>: This is true as well | + | |
| + | <math>\textbf{(D)}</math>: This is true as well. Since any game gives out a net total of two points, and there are <math>\binom{12}{2} = 66</math> games, there are a total of <math>132</math> points for any round-robin tournament. | ||
| + | |||
Therefore, answer choice <math>\boxed{\textbf{(E)}}</math> is false. If everyone ties, every team will be tied for the first place with 11 points each. | Therefore, answer choice <math>\boxed{\textbf{(E)}}</math> is false. If everyone ties, every team will be tied for the first place with 11 points each. | ||
| + | |||
| + | ~xHypotenuse | ||
== See Also == | == See Also == | ||
{{AMC12 box|year=2015|ab=A|num-b=12|num-a=14}} | {{AMC12 box|year=2015|ab=A|num-b=12|num-a=14}} | ||
Latest revision as of 14:17, 16 July 2024
Contents
Problem
A league with 12 teams holds a round-robin tournament, with each team playing every other team exactly once. Games either end with one team victorious or else end in a draw. A team scores 2 points for every game it wins and 1 point for every game it draws. Which of the following is NOT a true statement about the list of 12 scores?
Solution 1
We can eliminate answer choices 
and 
because there are an even number of scores, so if one is false, the other must be false too. Answer choice 
must be true since every team plays every other team, so it is impossible for two teams to lose every game. Answer choice 
must be true since each game gives out a total of two points, and there are 
games, for a total of 
points. Answer choice 
is false. If everyone draws each of their 11 games, then every team will tie for first place with 11 points each.
Quick question, why does the answer key say 3?
Solution 2
We will proceed by process of elimination:
: We know that this must be true, since any tied results in a 1 point (which is odd) for both teams. Hence, there must be 0 or a positive even number of odd scores.
: This is true too, because each non-tie generates 2 points for the winner, and 0 points for the loser, which are both even scores. Hence, there must be 0 or a positive even number of even scores as well.
: This must be true since every team plays every other team, so it is impossible for two teams to lose every game.
: This is true as well. Since any game gives out a net total of two points, and there are games, there are a total of points for any round-robin tournament.
Therefore, answer choice is false. If everyone ties, every team will be tied for the first place with 11 points each.
~xHypotenuse
See Also
| 2015 AMC 12A (Problems • Answer Key • Resources) | |
| Preceded by Problem 12 |
Followed by Problem 14 |
| 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 | |
