A batch of 50 different automatic typewriters contains exactly 10 defective machines. What is the probability of finding:
(a) At least one defective machine in a random group of 5 machines?
(b) At least two defective machines in a random group of 10 machines?
(c) The first defective machine to be the k-th machine taken apart for inspec- tion in a random sequence of machines?
(d) The last defective machine to be the k-th machine taken apart?