I do NOT want to know what would happen otherwise...

At the basement of a building with 5 floors, Adam, Bob, Cindy, Diana and Edward entered the elevator. The elevator goes only up and doesn’t come back, and each person gets out of the elevator at one of the five floors. In how many ways can the five people leave the elevator in such a way that at no time are there a male and a female alone in the elevator?


Did anybody get the joke in the title? :D


The answer is 1973.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Pawan Kumar
Mar 10, 2015

Total number of ways in which 5 5 people can leave at 5 5 floors = 5 5 5^5

For the exceptional ways of leaving the elevator, we have to consider all the ways in which a pair (a M and a F) can leave at the upper floors while remaining 3 3 people have left the elevator at the lower floors.

There are 6 6 ways to choose a pair out of 3 3 M and 2 2 F.

For a given pair, the number of exceptional ways in which they can leave are as follows:

case 1 1 :

Total exceptional ways in which each member of pair leaves at 5 t h 5^{th} floor =

Number of ways in which the remaining 3 3 people can leave at 4 t h 4^{th} to 1 s t 1^{st} floors = 4 3 4^{3}

case 2 2 :

Total exceptional ways in which each member of pair leaves at 5 t h 5^{th} to 4 t h 4^{th} floor such that at least one of them leave at 4 t h 4^{th} floor =

Number of ways in which the remaining 2 2 people can leave at 5 t h 5^{th} to 4 t h 4^{th} floors (excluding ways already considered in case 1 1 ) times Number of ways in which the remaining 3 3 people can leave at 3 r d 3^{rd} to 1 s t 1^{st} floors = 3 × 3 3 3 \times 3^{3}

case 3 3 :

Total exceptional ways in which each member of pair leaves at 5 t h 5^{th} to 3 r d 3^{rd} floor such that at least one of them leave at 3 r d 3^{rd} floor =

Number of ways in which the remaining 2 2 people can leave at 5 t h 5^{th} to 3 r d 3^{rd} floors (excluding ways already considered in case 1 1 and 2 2 ) times Number of ways in which the remaining 3 3 people can leave at 2 n d 2^{nd} to 1 s t 1^{st} floors = 5 × 2 3 5 \times 2^{3}

case 4 4 :

Total exceptional ways in which each member of pair leaves at 5 t h 5^{th} to 2 n d 2^{nd} floor such that at least one of them leave at 2 n d 2^{nd} floor =

Number of ways in which the remaining 2 2 people can leave at 5 t h 5^{th} to 2 n d 2^{nd} floors (excluding ways already considered in case 1 1 , 2 2 and 3 3 ) times Number of ways in which the remaining 3 3 people can leave at 1 s t 1^{st} floors = 7 × 1 3 7 \times 1^{3}

Total number of ways = 5 5 6 ( 4 3 + 3 × 3 3 + 5 × 2 3 + 7 × 1 3 ) = 1973 5^{5} - 6(4^{3} + 3 \times 3^{3} + 5 \times 2^{3} + 7 \times 1^{3}) = 1973

PS: Sorry for the typos (if any).

Why is the total number of ways 5 5? I would think it is 5!. For 2 peple and 2floors it isn't 2 2. It is 2

Greg Grapsas - 3 years, 7 months ago

Log in to reply

Not 55 in my comment 5 to the 5th power.

Greg Grapsas - 3 years, 7 months ago

Log in to reply

And 2 raised to the 2nd power not 22

Greg Grapsas - 3 years, 7 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...