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# permutation and combination

MA Economics Entrance Coaching Forums Subject forums Math permutation and combination

This topic contains 5 replies, has 3 voices, and was last updated by  rohit pandey 3 years ago.

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• #525

shruti290
Participant

q1 a seven digit no divisible by 9 is to be formed by using 7 no’s out of {1,2,3,4,5,6,7,8,9} the no of ways in which this can be done is? final ans:4*7!
q2 the no f ways in which 3 girls and 3 boys all of diff heights be arranged in a line so tht the boys as well as girls among themselves are arranged in decreasing order of their heights(frm left to right)?
final ans:20

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• #542

rohit pandey
Participant

shruti the answer for q2 is indeed 20 but i will have to explain it in class.hint -try forming a tree its really big.

and for q1 if you take and sum the different permutations of 7 numbers form set you will find the sum of 4 options is divisible by 9 hence 4*7!.

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• #544

shruti290
Participant

Rohit I got q2 but the 1st one i don’t know how to proceed.

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• #546

Keymaster

Rohit/ Shruti, if possible, can either of you upload your soln to q2 here? (take a photo)
For q1 sort of questions, see this for divisibility rules

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• #549

shruti290
Participant

I’m so sorry mh questions got interchanged in the reply ! I don’t know how to go abt for tht q2

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• #554

rohit pandey
Participant

this is if first choice is g1 which has ten ways now for b1 also there willbe 10 ways.

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