OOOO Different coloured pegs, each of which is painted in one and only one of the six colours red, white, black, green, blue and yellow, are to be placed in four holes, as shown in the figure, with one peg in each hole. Pegs of the same colour are indistinguishable. Calculate how many different arrangements of pegs placed in the four holes so that they are all occupied can be made from

(a) six pegs, all of different colours,

(b) two red and two white pegs,

(c) two red, one white and one black peg,

(d) twelve pegs, two of each colour. (L. Additional)