If there are no restrictions, there can be;
6 ways x 6 ways x 6 ways x 6 ways x 6 ways x 6 ways = 6^6 ways for the letters a, b, c, d, e and f to be arranged.
However, if you want to arrange them so that each letter is used only once;
6 ways x 5 ways x 4 ways x 3 ways x 2 ways x 1 way = 6! ways
Hope I helped :)
