Example

Let's say you are sorting mail, you have 5 letters from a credit card company, one for each person and 5 mailboxes on the wall. How many ways are there for every person to not recieve the incorrect offer from the credit card company? Assuming every offer is tailored to each individual.

To answer this you first need to recognise what sort of problem it is. It is easily distinguished as a derangement problem because the question asks about the number of ways to do something incorectly. Incorrect and wrong are keywords you shoud look for to distinguish a derangement problem.

Secondly, to solve the problem you need to recognise that there are 5 distinct letters in the problem so in the equation n = 5 distinct objects. Now you only need to use the formula and solve.

So there are 44 different ways you could get everything wrong.

Continue to a problem you can try!

Author: Chris Reeson