posted by robyn .
Imagine that you have 10 CDs and 10 envelopes on a table before you; each CD and each envelope are labelled with a number such as 7 or 10. If you randomly put the CDs into the envelopes provided, what is the probability of matching only one CD into the wrong envelope?
It took me a few minutes to realize this is a trick question. It is impossible to have exactly one CD in the wrong envelope. If a CD is in the wrong envelope, then there must be at least one other CD in a wrong envelope. So, the probability is zero.