permutation

 

Suppose we count a word as having 3 letters. How many "words" can be made from the 26 letters in the English alphabet? 

We know that this is permutation (order matters)  since "its" and "tis" are different words. Note the formula for permutations is n!(n−r)!n!(n−r)! using 'n’ different things taken ‘r’ at a time