In Exercises 21-26, a fair coin is tossed two times in succession.
The set of equally likely outcomes is {HH, HT, TH, TT). Find the
probability of getting
21. two heads.
22. two tails.
23. the same outcome on each toss.
24. different outcomes on each toss.
25. a head on the second toss.
26. at least one head.
