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I'm in a college Prob/stats course and I need help with how to find the answer to this: Jeanie is a bit forgetful, if she doesn't make a to do list the probability that she forgets something is 0.1. Tomorrow she has three errands and she fails to write them down. What is the probability that Jeanie remembers the first errand but not the second or third? The book gives the answer which is 0.999, but I don't understand how to get to that. Can someone explain it for me? Thanks in advance!

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  • $\begingroup$ Are you sure it's not 0.009? If she has a probability of only 0.1 of forgetting something, the probability of forgetting 2 out of 3 should be quite low. $\endgroup$ – Bill the Lizard Oct 6 '15 at 20:11
  • $\begingroup$ Oh gosh I posted the wrong part of the question! That would be 0.009. I meant to ask what is the probability that Jeanie remembers at least one of the three errands? $\endgroup$ – McCall Oct 6 '15 at 20:12
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I meant to ask what is the probability that Jeanie remembers at least one of the three errands?

This is the complement of Jeanie forgetting all three errands, so the answer is:

$1 - 0.1 * 0.1 * 0.1 = 0.999$

This is the easy way of solving it. You could also add up the probabilities that Jeanie forgets one, two or all three errands, and you should get the same result.

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  • $\begingroup$ Thank you. I didn't realize that would be the complement. $\endgroup$ – McCall Oct 6 '15 at 20:31

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