Used photocopying machines are returned to the supplier, cleaned, and then sent back out on lease agreements. Major repairs are not made, and, as a result, some customers receive malfunctioning machines. Among eight used photocopiers in supply today, three are malfunctioning. A customer wants to lease four of these machines immediately. Hence four machines are quickly selected and sent out, with no further checking. Find the probability that the customer receives
a No malfunctioning machines.
b At least one malfunctioning machine.
c Three malfunctioning machines.