Consider the alphabet A, B, C, D, E, F and make words without repetition of letters allowed.
(a) How many six-letter words are there?
(b) How many words begin with D or E?
(c) How many words end in B or A?
(d) How many words begin with D or E and end in B or A?
(e) How many have first letter neither D nor E and last letter neither B nor A?