Originally Posted by

**MadisonDuBerlais** Hello forum,

My names Frank and I've just signed up! I am currently struggling with my Probability especially Bayes theorem.

My first question is to find out what the probability of a letter that does not arrive the next day is 2nd class. The data I have available too me is;

1. 70% of Mail carries 1st class stamp.

2. 90% of 1st class mail arrives at its destination the next day.

Using Bayes Theorem

P(Letter Does Not Arrive/2nd Class) = 0.10 * 0.30

/

P(Letter Arives * 1st Class) + P(Letter does not arrive * 1st Class) = 0.72

ANS = 0.03/0.72 = 0.041

I know the ANS is 0.632 but I just don't know how to get there!