Need help : Permutation and combinations problem.

Need help : Permutation and combinations problem.

Postby Mahmoud Ibrahim » Sun Jun 03, 2018 4:55 am


The student should answer 10 questions out of 13 questions on a condition that he should
answer 4 questions at least from the first 5 questions. How many ways can he answer?
(A) 140 (B) 196 (C) 280 (D) 346
The correct answer is (B) 196 . Please will you explain how to get to this answer. Thanks.
Mahmoud Ibrahim
 
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Re: Need help : Permutation and combinations problem.

Postby phw » Mon Jun 04, 2018 5:29 am

I think you need to consider 2 cases.

If you answer 5/5 in the first section and 5/8 in the second, then there are 5C5 * 8C5 ways to choose which questions to answer.

If you answer 4/5 in the first section and 6/8 in the second, then there are (5C4) * (8C6) ways to choose which questions to answer.

[tex]{5 \choose 5}{8 \choose 5} + {5 \choose 4} {8 \choose 6} = 1\cdot56 + 5\cdot28 = 196.[/tex]

phw
 
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Re: Need help : Permutation and combinations problem.

Postby Guest » Thu Jun 07, 2018 9:21 am

Thank you very much. Many many thanks.
Guest
 


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