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?

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Answer 1 · 2015-01-02T23:31:16

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.

So add: #P(3 or 4)=P(3)+P(4)=0.1+0.1= 0.2 #

Check your answer (if you have time left during your test), by calculating the opposite probability:

#P(<3)=P(0)+P(1)+P(2)=0.1+0.3+0.4=0.8#
And this and your answer add up to #1.0#, as they should.