Find the probability of A occurring given that B occurs.
Use a tree diagram to work out the probabilities.
Multiply when going deeper into a tree…
Add when combining separate branches….
So probability of getting a Q would be…
P(Q) = (1/3) . (1/2) + (2/3) . (1/4) = 1/6 + 1 / 6 = 1/3
Example from patrickJMT