Saturday, March 10, 2018

Mismatched Letters

Soon after her wedding, a young bride hand-writes 100 thank you letters to all of her recent wedding guests and addresses 100 matching envelopes. Being in a hurry to get them in the mail, her new husband randomly stuffs the letters into envelopes and mails them out. What is the probability that exactly 99 of the letters made it into the right envelope?

1 comment:

John Johnson II said...

Haha, very clever. I didn't make that observation, so I tried using the multiplication rule of probability: P(A) and P(B)= P(A)*P(B). Which gave me (1/100)^99. Although the solution shows that isn't the answer. This is one particular case of why probability can be hard to check validity at least for beginners like me.