Difference between revisions of "2005 AMC 10A Problems/Problem 18"
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Revision as of 11:38, 10 August 2022
Problem
Team A and team B play a series. The first team to win three games wins the series. Each team is equally likely to win each game, there are no ties, and the outcomes of the individual games are independent. If team B wins the second game and team A wins the series, what is the probability that team B wins the first game?
Solution
There are at most 
games played.
If team 
won the first two games, team 
would need to win the next three games. So the only possible order of wins is 
.
If team won the first game, and team won the second game, the possible order of wins are: 
and 
, where 
denotes that the th game wasn't played.
There is 
possibility where team wins the first game and 
total possibilities when team wins the series and team wins the second game. Note that the fourth possibility 
occurs twice as often as the others, so we put over total possibilities. The desired probability is then 
.
counts as two cases because it can be either 
or 
.
~lnzhonglp
See Also
| 2005 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 17 |
Followed by Problem 19 | |
| 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. 
