How many different two letter arrangements can you make without repeating letters?

Answer 1

If you have an alphabet of 26 letters, you can make

26 x 25 = 650 arrangements if "nt" is considered different from "tn" (order counts)

or

26 x 25 / 2 = 325 arrangements if "nt" is considered the same as "tn" (order doesn't count)

The permutation and combination formulas for 26 objects where 2 are chosen are

26! / (26-2)! for order counts (permutation)

26! / (26-2)!2! for order doesn't count (combination)