A bag contains tennis balls:
of the balls are yellow, the other balls are white
John will take at random a ball from the bag.
He will note its colour and then put it back in the bag.
Then Mary will take at random a ball from the bag.
She will note its colour.
Given the probability than John and Mary will take balls with different colours is ,
. . prove that: .
We are told that this probability is
Divide by 2: .