Not in Book 3 : Float like a Butterfly, String like a Bee [3 points] How many strings of six uppercase English letters are there:

a) if letters can be repeated?

b) if no letter can be repeated?

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

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