A bag contains 4 white marbles and 3 blue marbles. Another bag contains 5 red marbles and 6 blue marbles. If a marble is picked at random from each bag, find the probability that they are of the same colour.
Worked solution (try it first)
The first bag has white and blue.
The second has red and blue.
So the only colour they share is blue.
Both blue: 73×116, since the picks are independent.