How many strings of eight uppercase English letters are there

a) if letters can be repeated?

b) if no letter can be repeated?

c) that start with X, if letters can be repeated?

d) that start with X, if no letter can be repeated?

e) that start and end with X, if letters can be repeated?

f) that start with the letters BO (in that order), if letters can be repeated?

g) that start and end with the letters BO (in that order), if letters can be repeated?

h) that start or end with the letters BO (in that order), if letters can be repeated?