Question

Find the number of permutations of the last four digits in the phone number 555-1234.

Answer 1

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you have to fill digits 1,2,3,4 in these spaces
now in first one you can put any of these 4 digits,,so no. of ways = 4
in the next space,,you have 3 nos left as 1 has already been used,,so no. of ways =3
for next its 2
and then its 1
so total no. of ways = 4*3*2*1 = 24