You have studied the number of people waiting in line at your bank on Friday afternoon at 3 pm for many years, and have created a probability distribution for 0, 1, 2, 3, or 4 people in line. The probabilities are 0.1, 0.3, 0.4, 0.1, and 0.1, respectively. What is the probability that at least 3 people are in line at 3 pm on Friday afternoon?
This is an EITHER... OR situation. You may ADD the probabilities.
The conditions are exclusive , that is: you can't have 3 AND 4 people in a line. There are EITHER 3 people OR 4 people in line.
Check your answer (if you have time left during your test), by calculating the opposite probability:
And this and your answer add up to