another probability problem

A company has 8 telephone lines. If 4 calls have to be made by the company, what s the probability that they are each made on a different line?
(A)35/2048
(B)105/256
(C)105/2048
(D)35/256
(E)105/128

This one also confused the daylites outta me. Hope I can have a detailed explanation. OA is B

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Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 am; Location: India; Thanked: 375 times; Followed by:53 members

by Frankenstein » Thu Jun 23, 2011 6:04 am

Hi,
Each call can be made from any of the given 8 lines in 8 ways.
So, total number of ways is 8^4
In order to make calls from 4 different lines,
1st call has 8 lines to pick from
2nd call has remaining 7 to pick from
3rd has 6
4th has 5
So, probability is 8.7.6.5/8^4 = 105/256

Cheers!

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