American Express Business Travel uses a six-letter record locator number (RLN) for each client's trip (e.g., KEZLFS).
(a) How many different RLNs can be created using capital letters (A­Z)?
(b) What if they allow any mixture of capital letters (A­Z) and digits (0­9)?
(c) What if they allow capital letters and digits but exclude the digits 0 and 1 and the letters O and I because they look too much alike?