# Conditional Probability of computer password

1. Feb 8, 2012

### lina29

1. The problem statement, all variables and given/known data
Two computers are connected to a password-protected wireless network. When the password is temporarily removed, a virus can attack the first computer with probability 0.5, the second computer with probability 0.7, and it can attack both computers with probability 0.4.

The first computer was not attacked at all. What is the probability that the second computer was attacked?

2. Relevant equations

3. The attempt at a solution
I assume I need the find P(B|complement of A)= P(B^ complement of A)/complement of A
so I got .7/.5= 1.4 and I know that's wrong since probabilities can't be over 1
Am I supposed to do .3/.5= .6 instead?

2. Feb 8, 2012

### Dick

Yes, .3/.5. P(B^complement A)=0.3, right?

3. Feb 8, 2012

### lina29

Thank you! I have another conditional probability question that I need help with. Am I allowed to post it on this thread or did I have to make another one?

4. Feb 8, 2012

### Dick

You could. It's not against the rules. Can't say I'll be here to answer it though. Might be better to put it in a new thread. That way it will have zero answers and it will get more attention.

Last edited: Feb 8, 2012
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