1% of women at age forty who participate in routine screening have breast cancer.  80% of women with breast cancer will get positive mammographies.  9.6% of women without breast cancer will also get positive mammographies.  A woman in this age group had a positive mammography in a routine screening.  What is the probability that she actually has breast cancer?

What do you think the answer is?  If you haven't encountered this kind of problem before, please take a moment to come up with your own answer before continuing.

http://yudkowsky.net/bayes/bayes.html

it should be clear if we simply draw the diagram to represent the relationship...

             80% have cancer  ( positive)      
1%  ----<
             20% have cancer   (nagative)

      
             9.6% no cancer ( positive)
99%----<
             93.4% no cancer ( nagative)

then ? whats next >? I got confused ...

if the result is positive... she has 0.01*0.08 / ( 0.01*0.08 + 0.99*0.096)
.. am I right ? almost the conditional probability ...

If you keep reading on that website you will have a very clear picture.