A store sells 7 different types of items: mugs, t-shirts, books, postcards, hats, socks and sweaters. There are 6 customers in the store and they each buy one type of item at random. Calculate the probability that a) The customers each purchase a different type of item. (1 mark) b) No customer purchases books or postcards. (1 mark) (3) At least four customers purchase a mug. (2 marks) d) Three customers purchase the same type of item and the other three customers purchase another same type of item. (2 marks)