Difference between revisions of "2013 AMC 10A Problems/Problem 24"
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There are <math>1+1+2\cdot2\cdot2\cdot1 = 10</math> ways to assign opponents to <math>B</math> and <math>90\cdot10 = 900</math> ways to order the games. <math>\boxed{\textbf{(E)} 900}</math> | There are <math>1+1+2\cdot2\cdot2\cdot1 = 10</math> ways to assign opponents to <math>B</math> and <math>90\cdot10 = 900</math> ways to order the games. <math>\boxed{\textbf{(E)} 900}</math> | ||
==Video Solution by Richard Rusczyk== | ==Video Solution by Richard Rusczyk== | ||
| − | www.youtube.com | + | https://www.youtube.com/watch?v=8rxohCt2uQc |
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~dolphin7 | ~dolphin7 | ||
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==See Also== | ==See Also== | ||
Revision as of 15:20, 2 April 2020
Problem
Central High School is competing against Northern High School in a backgammon match. Each school has three players, and the contest rules require that each player play two games against each of the other school's players. The match takes place in six rounds, with three games played simultaneously in each round. In how many different ways can the match be scheduled?
Solution 1
Let us label the players of the first team 
, 
, and 
, and those of the second team, 
, 
, and 
.
. One way of scheduling all six distinct rounds could be:
Round 1---->
Round 2---->
Round 3---->
Round 5---->
Round 2---->
Round 3---->
Round 5---->
The above mentioned schedule ensures that each player of one team plays twice with each player from another team. Now you can generate a completely new schedule by permutating those 
rounds and that can be done in 
ways.
. One can also make the schedule in such a way that two rounds are repeated.
(a)
Round 1---->
(b)
Round 1---->
As mentioned earlier any permutation of (a) and (b) will also give us a new schedule. For both (a) and (b) the number of permutations are
= 
So the total number of schedules is 
=
.
Solution 2
Label the players of the first team , , and , and those of the second team, , , and . We can start by assigning an opponent to person for all games. Since has to play each of , , and twice, there are 
ways to do this. We can assume that the opponents for in the rounds are , , , , , and multiply by afterwards.
Notice that for every valid assignment of the opponents of and , there is only 
valid assignment of opponents for . More specifically, the opponents for are the leftover opponents after the opponents for and are chosen in each round. Therefore, all we have to do is assign the opponents for . This is the same as finding the number of permutations of , , and that do not have a in the first two spots, an in the next two spots, and a in the final two spots.
We can use casework to find this by using the fact that after we put down the 's and 's first there is way to put down the 's (the two remaining spots).
If 's are put in the middle 
spots, then there is way to assign spots to , namely the last spots. (If one of the last two spots are left empty, there will have to be a there, which which not valid).
If 's are put in the last spots, then there is way to assign spots to .
Finally, if one was put in on of the middle two spots and one was put in one of the last two spots, there are 
ways to assign spots to and 
ways to assign spots to (one of the first two spots and the remaining spot in the last ).
There are 
ways to assign opponents to and 
ways to order the games.
Video Solution by Richard Rusczyk
https://www.youtube.com/watch?v=8rxohCt2uQc
~dolphin7
See Also
| 2013 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 23 |
Followed by Problem 25 | |
| 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 | ||
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. 
