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**1. You give a friend a letter to mail. He forgets to mail it with probability 0.2. Given that**

he mails it, the Post Office delivers it with probability 0.9. Given that the letter was not

delivered, what’s the probability that it was not mailed?

he mails it, the Post Office delivers it with probability 0.9. Given that the letter was not

delivered, what’s the probability that it was not mailed?

**2. I assume I'm supposed to use Bayes Formula, but I'm confused as to what we really know in order to solve the problem.**

**3. I used A= not mailed, B= not delivered. I have this equation, P(A given B) = (P(B given A) *P(A))/ P(B given A)*P(A) + P(B given A complement)*P(A complement)**

which equals (?*.20)/?*.20 + ? * .80

which equals (?*.20)/?*.20 + ? * .80

Help please.