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Sixteen players S1, S 2 . . . . . . . . . . . . . . . . . . . S 16 play in a tournament. They are divided into eight pairs at random. From each pair a winner is decided on the basis of a game played between the two players of the pair. Assume that all the players are of equal strength (a) Find the probability that the player S 1 is among the eight winners. (b) Find the probability that exactly one of the two players S 1 and S 2 is among the eight winners.

Sixteen players S1, S2 . . . . . . . . . . . . . . . . . . . S16 play in a tournament. They are divided into eight pairs at random. From each pair a winner is decided on the basis of a game played between the two players of the pair. Assume that all the players are of equal strength
(a) Find the probability that the player S1 is among the eight winners.
(b) Find the probability that exactly one of the two players S1 and S2 is among the eight winners.

Grade:11

2 Answers

Aditi Chauhan
askIITians Faculty 396 Points
10 years ago
Hello Student,
Please find the answer to your question
(a) Prob. of S1 to among the eight winners = (prob. of S1 being in a pair) x (prob. of S1 winning in the group).
= 1 x 1/2 [∵ S1 is definitely in a group i.e. certain event] = 1/2
(b) If S1 and S­2 are in the same pair then exactly on wins. If S1 and S2 are in two pairs separately then exactly one of S1 and S2 will be among the eight winners if S1 win and S2 loses or S1­ loses and S2 wins.
Now the prob. of S1, S2 being in the same pair and one wins.
= (prob. of S1, S2 being in the same pair) x (prob. of any one winning in the pair)
And the prob. of S1, S2 being in the same pair
= n (E)/n (S), where n (S) = the no. of ways in which 16 person can be divided in 8 pairs; n (E) = the no. of ways in which S1, S2 are in same pair or 14 persons can be divided into 7 pairs.
∴ n (E) = 14!/(2!)7 .7! and n (S) = 16!/(2!)8 .8!
∴ Prob. of S1 and S2 being in the same pair
= \frac{\frac{14!}{(2!)^7 7!}}{\frac{16!}{(2!)^8 .8!}}= 21.8/16.15 = 1/15
The prob. of any one winning in the pair of S1, S2 = P (certain event) = 1
∴ The pair of S1, S2 being in two pairs separately and any one of S1, S2 wins.
= the prob. of S1, S2 being in two pairs separately and S1 wins, S2 loses + the prob. of S1, S2 being in two pairs separately and S1 loses, S2 wins.
= \left [ 1-\frac{\frac{14!}{(2!)^7 7!}}{\frac{16!}{(2!)^8 .8!}}\right]x 1/2 x 1/2 + \left [ 1-\frac{\frac{14!}{(2!)^7 7!}}{\frac{16!}{(2!)^8 .8!}}\right]x 1/2 x 1/2
= 236-2051_1.jpgx 1/4 = 1/2 x 14 x 14!/15 x 14! = 7/15
∴ Required prob. = 1/15 + 7/15 = 8/15

Thanks
Aditi Chauhan
askIITians Faculty
Sher Mohammad IIT Delhi
askIITians Faculty 174 Points
10 years ago
Since the probablity of winning each is is ½, so the probablity of s1 win is ½.
the second probablity is ( 1/15 ) + ( 14/15 ) ( ½ ) = 8/15
sher mohammad
faculty askiitians
b.tech, iit delhi

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