# probability tree diagram alevel year 2

Hey guys I'm back with a second probability question:

https://www.mathsgenie.co.uk/alevel/s1jan2011ans.pdf

I'm stuck on question 7a, I don't understand how they figured out the probability for each direction. Is the second section of branches supposed to be Bag Q? Or is it the third section? Genuinely just don't understand the whole diagram and how they got the numbers.
Original post by cherrycosmos
Hey guys I'm back with a second probability question:

https://www.mathsgenie.co.uk/alevel/s1jan2011ans.pdf

I'm stuck on question 7a, I don't understand how they figured out the probability for each direction. Is the second section of branches supposed to be Bag Q? Or is it the third section? Genuinely just don't understand the whole diagram and how they got the numbers.
Its the usual (conditional) probabilities, so the first two levels are drawing the colour from bag P and the third level is from bag Q, as the question states. So for the top branch, first level
p(red from P) = 3/6 = 1/2
Second level
p(red from P | you drew a red from P and placed in Q) = 2/5
Third level
p(red from Q | you drew 2 reds from P and placed in Q) = 6/9
(edited 1 month ago)