5 men and 7 women are standing in a queue. Let n be the number of ways in which all 7 women stand together. Let m be the number of ways in which exactly 6 women stand together. Find
Note : All 12 people stand in both cases and no 2 people are identical.
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We are required to find n m
Case 1: to find n
There are 7 women. So let us consider all 7 together as 1. Then the total number of people standing in the queue would be 6 ( 5 men + 1). They would arrange themselves with 6 ! .And the 7 ladies would arrange themselves as 7 !
By multiplication rule
n = 6 ! × 7 !
Case 2: to find m
First we need to select 6 women out of 7. It happens by ( 6 7 ) . These 6 women arrange in 6 ! ways.Thus total is 7 ! . Then , let us consider these 6 women as x , the 7th woman as y . Clearly there are 7 people in the line to stand x + y + 5 .
m = total number of ways to stand for 7 people - the situations in which x and y stand next to each other ( when x and y stand next to each other the case of m is violated)
Total number of ways = 7 ! × 7 !
_ _ _ _ _ _ _ Consider these 7 dashes for the 7 people to stand. There are 6 × 2 ways for x and y to stand together. The 5 men arrange themselves as 5 !
Situation in which m case is violated = 7 ! × 1 2 × 5 !
Thus m = ( 7 ! × 7 ! ) − ( 7 ! × 1 2 × 5 ! )
Therefore n m = 7 ! × 6 ! ( 7 ! × 7 ! ) − ( 7 ! × 1 2 × 5 ! ) ⟹ n m = 5