A mail order company employs three stock clerks, A,B and C, who pull items from shelves and assemble them for subsequent verification and packaging. A makes a mistake in an order (gets an item wrong or the wrong quantity) one time in a hundred, B makes a mistake in an order five times in a hundred and C makes a mistake in an order three times in a hundred. If A, B and C fill respectively 30, 40 and 30 percent of all orders, what are the probabilities that
a) a mistake will be made in an order.
b) if a mistake is made in an order, it was filled by A?
c) if a mistake is made in an order, it was filled by B?