I should start by saying I am a mature student (embarrassed! ) and it isn't strictly speaking homework but I am using the AQA website to work through past papers as I am self taught. I am struggling with a probability question and I find that using probability trees helps me. Well, the question is:
You have 100 people divided into 14 male managers and 26 male teachers, 8 female managers and 52 female teachers. What is the probability that you will select a male teacher and any female.

On my "tree" I have put the probability of males / females is 2/5 and 3/5 respectively.
I have then on my branch off the "males" I have divided males into 7/20 probability for manager and 13/20 probability for teacher. On my branch off the "females" I have divided females into 2/15 managers and 13/15 teachers.

I worked out then that probability of male teacher = 2/5 x 13/20 and any female = 3/5 x 2/15 plus 3/5 x 13/15

Needless to say my answer is wrong (AQA provide mark sheets). Help! :(

OK - so I did it again - the question asks for the answer in tree form so: I re-did it as male teachers = 26/100 (13/50) multiplied by "any female" = 60/99 (20/33) = 260/1650 or 26/165

They give the answer as the above but multiplied by 2. Where does the 2 come from. Please help because I'm tearing my hair out.:o

OK - so I did it again - the question asks for the answer in tree form so: I re-did it as male teachers = 26/100 (13/50) multiplied by "any female" = 60/99 (20/33) = 260/1650 or 26/165

They give the answer as the above but multiplied by 2. Where does the 2 come from. Please help because I'm tearing my hair out.:o


Sorry - been having a "bad hair day" - went away, had a cup of coffee, worked it out with help from another forum! Forgot to multiply by 2 because they could have selected a female first. :p