Ten minutes after the salesperson has started working, he made a sale. What is the probability that this is the only sale that the salesperson will make within the first hour?

This is a bit tricky. ... Let's rephrase the question in a way that will make it easier to translate to the language of probability. We are given that one sale has been made 10 minutes into the hour. This means that the number of sales that will be made within the hour, X, is at least one. In other words, we are given that X is greater than or equal to 1. Given that information, we are asked to find the probability that this will be the only sale, i.e., that X = 1. Putting this together, the question asks you to find: P(X=1 | X ≥ 1).