5.51 An appliance comes in two colors, white and brown, which are in equal demand. A certain dealer in these appliances has three of each color in stock, although this is not known to the customers. Customers arrive and independently order these appliances. Find the probability that a The third white is ordered by the fifth customer.

b The third brown is ordered by the fifth customer.

c All of the whites are ordered before any of the browns.

d All of the whites are ordered before all of the browns.