1. Suppose one has 20 indistinguishable balls. How many ways can one put them in 4 distin- guishable boxes, where one or more boxes may also be empty?

2. You type a document with N characters on your laptop. The probability that a single char- acter is typed correctly is p, where 0 < p < 1, independently of all other characters. Calculate

the probability of there being at most two errors in the entire document.