How many different 4-letter radio station call letters can be
made
a. if the first letter must be C, W, P, or N and no letter may be
repeated?
b. if repeats are allowed (but the first letter is C, W, P, or
N)?
c. How many of the 4-letter radio station call letters (starting
with C, W, P or N) have no repeats and end with the letter R?
a. If the first letter must be C, W, P...