 A simple example: a ball is picked at random. 70% of the balls are red (so the probability that a red ball is picked is 70%). 60% of the red balls have a black spot (so the probability that a ball with a black spot is picked, on the supposition that a red ball is picked, is 60%). The probability that a red ball with a black spot is picked is 60% of 70%, i.e. 42%. 