Answer:
5,040 different passwords are possible
Step-by-step explanation:
Assuming that 4-digit means using only numeric values.
This is a permutations problem.
n = number of values used with number values only this is 10
r = the number used in each case without repeating in this case 4
nPr = n! / (n - r)!
nPr = 10! / (10-4)!
nPr = 10! / 6!
The ! means factorial. For example 4! = 4 x 3 x 2 x 1 = 24
A quick hack for dividing factorials is to use cancelling.
10! / 6! = 10 x 9 x 8 x 7 (because 6 to 1 are cancelled by 6!)
= 5040
