Problem 40BB

Oldie but Goodie A secretary types 10 different letters and addresses 10 corresponding envelopes. If he is in a hurry and randomly inserts the letters into the envelopes, what is the probability that exactly 9 of the letters are in the correct envelopes? (Based on MathematicsMagazine, Vol. 23, No. 4, 1950)

Solution 40BB

= 3,628,800