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805+ (Hard)|   Combinations|      
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Bunuel
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In a group of 8 semifinalists, all but 2 will advance to the final rou 22 May 2016, 14:46
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35% (01:29) correct 65% (01:44) wrong based on 327 sessions

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In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals, then how many groups of medal winners are possible?

(A) 20
(B) 56
(C) 120
(D) 560
(E) 720
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B
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chetan2u
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GMAT Focus 1: 735 Q90 V89 DI81
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In a group of 8 semifinalists, all but 2 will advance to the final rou 22 May 2016, 20:26
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Bunuel
In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals, then how many groups of medal winners are possible?

(A) 20
(B) 56
(C) 120
(D) 560
(E) 720
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Hi,

there are two points to remember-


1) Is it important here that how many go into finals... NO....
what is important is HOW many get medals..3
finally these 3 can be any of the 8 - \(8C3 = \frac{8!}{5!3!} = \frac{8*7*6}{3*2}=56\)

2) Is order important...NO
we are looking for groups only..

ans 56
B
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dabhishek87
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In a group of 8 semifinalists, all but 2 will advance to the final rou 09 Jun 2017, 10:11
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For this problem we have to select 3 semifinalists out of 8 so
\(8_C_3\)
Answer B
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Palaksehgal3
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In a group of 8 semifinalists, all but 2 will advance to the final rou 03 Jul 2017, 02:15
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As per the question, only 6 semifinalists made it to the 3rd round.
So why is the answer not 6c3?
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In a group of 8 semifinalists, all but 2 will advance to the final rou 03 Jul 2017, 02:30
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Palaksehgal3
As per the question, only 6 semifinalists made it to the 3rd round.
So why is the answer not 6c3?
Show more

Yes, but initial number is still 8. Do we know which 2 won't advance to the final? No. So, the size of the group we are selecting from is still 8.

In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals, then how many groups of medal winners are possible?

(A) 20
(B) 56
(C) 120
(D) 560
(E) 720

Intermediary step about "all but 2 will advance to the final round" is just to confuse us: any group of 3 is equally likely to win, so we can skip this part and directly calculate ways to choose 3 contestants out of 8: \(C3_8=56\).

Answer: B.
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In a group of 8 semifinalists, all but 2 will advance to the final rou 03 Jul 2017, 02:42
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Bunuel
Palaksehgal3
As per the question, only 6 semifinalists made it to the 3rd round.
So why is the answer not 6c3?

Yes, but initial number is still 8. Do we know which 2 won't advance to the final? No. So, the size of the group we are selecting from is still 8.

In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals, then how many groups of medal winners are possible?

(A) 20
(B) 56
(C) 120
(D) 560
(E) 720

Intermediary step about "all but 2 will advance to the final round" is just to confuse us: any group of 3 is equally likely to win, so we can skip this part and directly calculate ways to choose 3 contestants out of 8: \(C3_8=56\).

Answer: B.
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Makes sense. Thanks Bunuel :)
I am having a hard time with probability and permutations. Could you please advise how to go about improving at it?
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In a group of 8 semifinalists, all but 2 will advance to the final rou 03 Jul 2017, 02:44
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Palaksehgal3
Bunuel
Palaksehgal3
As per the question, only 6 semifinalists made it to the 3rd round.
So why is the answer not 6c3?

Yes, but initial number is still 8. Do we know which 2 won't advance to the final? No. So, the size of the group we are selecting from is still 8.

In a group of 8 semifinalists, all but 2 will advance to the final round. If in the final round only the top 3 will be awarded medals, then how many groups of medal winners are possible?

(A) 20
(B) 56
(C) 120
(D) 560
(E) 720

Intermediary step about "all but 2 will advance to the final round" is just to confuse us: any group of 3 is equally likely to win, so we can skip this part and directly calculate ways to choose 3 contestants out of 8: \(C3_8=56\).

Answer: B.

Makes sense. Thanks Bunuel :)
I am having a hard time with probability and permutations. Could you please advise how to go about improving at it?
Show more

Check the links below:
Combinatorics Made Easy!

Theory on Combinations

DS questions on Combinations
PS questions on Combinations

Tough and tricky questions on Combinations

Probability Made Easy!

Theory on probability problems

Data Sufficiency Questions on Probability
Problem Solving Questions on Probability

Tough Probability Questions

Hope it helps.
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