Twenty balls are placed in an urn. Five are red, five green, five yellow and five blue. Three balls are drawn from the urn at random without replacement. Write down expressions for
the probabilities of the following events. (You need not calculate their numerical values.)
a) Exactly one of the balls drawn is red.
b) The three balls drawn have different colours.
c) The number of blue balls drawn is strictly greater than the number of yellow balls