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Permutation problem

(Guest)

Q1The number of ways in which 7boys sit in a round table so that two particular boys may sit together is

A]240

B]200

C]120

D]none of these

Q2 The number of arrangement of 10 different thing taken 4 at atime in which one particular thing always occur

Explain abovetwo through giving their solution.

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4 Replies

Ankit Hinduja (Student) (1227 Points)
Replied 23 February 2012

1. See the formula to find no. Of persons to be seated on round table is (n-1)!. Here , any two persons should always sit together , so consider these two as one object , so we are left with 7-1 = 6 objects. Now according to formula we have (6-1)! = 120.

Ankit Hinduja (Student) (1227 Points)
Replied 23 February 2012

2. The formula to find out no. Of permutations /arrangements when any one object is always included is (n-1)P(r-1).. Here n=10 , r=4.. So we have 9P3=9!/(9-3)!=504