There are three independent events with probabilities of 1/3, 2/7, and 1/5. Find the probability that: Exactly one event will happen.
Say that the first one does happen. What are the probabilities that the others 2 won't happen? (Do them seperately)
Multiply the three probs together.
Then do the same for the second one. etc
when you have done all three, add your answers together.