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MC II-B Problem 4.29 solution wrong ?

 
 
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MC II-B Problem 4.29 solution wrong ?
by Michael Lu - Thursday, January 11, 2018, 10:53 PM
 

Problem 4.29 Suppose some friends go to a party. They each wear a coat. However, as they are leaving, they each randomly grab a coat. How many ways can the friends leave so that none of them have their own coat, if there are 5 friends?

The week 4 solution says 76 computed as following

5!− 5! / 2! + 5! / 3! − 5! / 4! + 5! / 5! = 76.

But correct answer seems to be 5! - 76 = 44 .

76 is the number of ways at least one friend grabbed his/her own coat and 44 is the number of ways no one left with their own coat.

It's easy to verify there are only 2 ways when there are 3 friends which is NOT

3! - 3!/2! + 3!/3! =  4.  Instead 6 - 4 = 2 is correct.

Can someone confirm ?

Thanks.


 
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Re: MC II-B Problem 4.29 solution wrong ?
by Areteem Professor - Friday, January 12, 2018, 9:57 AM
 

Michael,

Thanks for the catch. Yeah, the pattern should be

$$5! - \frac{5!}{1!} + \frac{5!}{2!} - \frac{5!}{3!} + \frac{5!}{4!} - \frac{5!}{5!} = 44,$$

as you mentioned. The handout is now updated.

Thanks!