standing in a queue is easy, isn't it

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 m n \cfrac mn

Note : All 12 people stand in both cases and no 2 people are identical.


The answer is 5.

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1 solution

Utkarsh Kumar
Mar 15, 2018

I know the solution is long but It is worth reading (I feel so). Plz read it completely and tell me if there is a problem.

We are required to find m n \cfrac mn

Case 1: to find n 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 ! 6! .And the 7 ladies would arrange themselves as 7 ! 7!

By multiplication rule

n = 6 ! × 7 ! n = 6! \times 7!

Case 2: to find m m

First we need to select 6 women out of 7. It happens by ( 7 6 ) \dbinom 76 . These 6 women arrange in 6 ! 6! ways.Thus total is 7 ! 7! . Then , let us consider these 6 women as x x , the 7th woman as y y . Clearly there are 7 people in the line to stand x + y + 5 x+y+5 .

m = 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 ! = 7! \times 7!

_ _ _ _ _ _ _ \huge \text{\_ \_ \_ \_ \_ \_ \_} Consider these 7 dashes for the 7 people to stand. There are 6 × 2 6 \times 2 ways for x and y to stand together. The 5 men arrange themselves as 5 ! 5!

Situation in which m case is violated = 7 ! × 12 × 5 ! = 7! \times 12 \times 5!

Thus m = ( 7 ! × 7 ! ) ( 7 ! × 12 × 5 ! ) m = (7! \times 7!)-(7! \times 12 \times 5!)

Therefore m n = ( 7 ! × 7 ! ) ( 7 ! × 12 × 5 ! ) 7 ! × 6 ! \dfrac mn = \dfrac {(7! \times 7!)-(7! \times 12 \times 5!)}{7! \times 6!} m n = 5 \implies \boxed {\dfrac mn = 5}

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