that involve the phrase "at least 5"?

You can calculate the probability that 6 get lost, and repeat for 7, 8 & 9. Then add them up. This will give you an exact answer, but it becomes laborious if the question is about a mass mailout of 1,000 letters.

Or you can take a shortcut, and observe that because the probability that an individual letter gets lost is pretty low (0.1%), the probability that

*most* of the letters will get lost is

*extremely *low.

So you can approximate the probability of "more than 5 getting lost" as "the probability of 6 getting lost", knowing that this will be a slight underestimate.

Now, if the question were about a mass mailout of 10,000 letters, this approximation is no longer valid, since you expect about 10 to get lost.