This question was quite popular and well attempted. Candidates appear to have performed better in (a) than in (b).
They were able to understand that all of them wins
= X wins and Y wins and Z wins = ¼ x 2/5 x 3/5 = 3/50 or 0.06
In the part (b), they were also able to understand that one of them wins means
X wins and others do not win or Y wins and others do not win or Z wins and others do not win = ¼ x 2/5 x 3/5 + ¾ x2/5 x 2/5 + ¾ x 3/5 x 3/5 = 6/100 x 12/100 x 27/100 = 9/20 or 0.45
