# Conditional probability help

1. Jan 20, 2014

### Retro95

I can easily solve a problem if the probabilities are already defined in the question.. such as P(A|B)
But I always get confused about how to actually define the probability from an application exercise.

E.g..
Market research shows that (A)75% if customers want text messages on their phone, (B)80% want photo capability, (AnB)65% want both.

What are the probabilities that a person who wants text messages also wants photo capability?

Would the formula be
or

Also could you please explain in detail how I would know which letter goes first since I always get it confused.

2. Jan 20, 2014

### mathman

P(B|A), B is the variable which has the probability. A is the condition.
Your expression is a little imprecise the way it is worded. A is the event a customer wants text mesages and P(A) = .75, etc.

In formal terms you are asking what is the probability that a person wants photo capability under the condition the person wants text messages.

3. Jan 20, 2014

### Retro95

Yeah thanks I eventually figured it out after a lot of application question practice.
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Also I have an exam in 2 days but my stupid lecturer decided not to give us the answer sheet to a previous past paper for us to practice and check our answers on.

So basically I have a past question paper PDF and I'm willing to pay someone to answer it 100% accurately so that I can see which parts I need to improve on before the exam

Inbox me or reply to this thread if anyone reading this is extremely confident in the following topics..
Descriptive Statistics,
Probability, Probability Distributions,
Sampling and Sampling Distributions,
Interval Estimation,
Hypothesis Tests