+614 Hong, the mailperson, delivers 5 letters to five houses. what is the probability she delivers at least one letter correctly?
+1224 Count the probability that she delivers all the letters incorrectly. This is the number of derangements of 5, which is 44.
Probability: 1 - 44/120 = 19/30
Read more about derangements here: https://en.wikipedia.org/wiki/Derangement
