need help with 25 and 29th

25. The set of strings over (a, b, c) in which every b is immediately followed by at lea one c. 26. The set of strings over la, bl in which the number of a's is divisible by three. 27. The set of strings over (a, b, c) in which the total number of b's and c's is three. 28. The set of strings over (a, bl in which every a is either immediately preceded immediately followed by b, for example, baab, aba, and b. 29. The set of strings over {a, b, c} that do not contain the substring aa. 0 The set of strines over (a, bl that do not begin with the substring gaa