Probability and statistics

Question

How many four-letter permutations can be formed from The first five letters of the alphabet?

Answer 1

nPr = n!/(n-r)!

5P4 = 5!/(5-4)! = 5!/1! = 5x4x3x2 = 120 permutations of 4 letters from 5 letters

combinations are

5C4 = 5!/4!1! = 5

if order doesn't matter there are only 5 combinations, each one leaving out one of the 5 letters

ABCD leave out E

ABCE leave out D

ABED leave out C

ACDE leave out B

BCDE leave out A

for each of the 5 there are 4! permutations = 4x3x2x1= 24

24x5 = 120 permutations total