Question

How many permutations exist of the letters a, b, c, d taken four at a time?

Answer 1

im not too sure...but i would do this
4(4x3x2x1)

Answer 2

There are 4 choices for the first letter. Having chosen the first letter there are 3 choices for the second letter. Having chosen the first and second letters, there are 2 choices for the third letter. Having chosen the first three letters there is only one letter remaining,
Therefore the number of permutations of the 4 letters taken four at a time is:
\[\large P(4, 4)=4\times3\times2\times1=you\ can\ calculate\]