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Originally Posted by suprovo2000 Five different letters are put at a random inside five addressed envelopes.Find the probability of putting exactly 2 letters in the correct envelopes.
Please tell me this answer in details properly. |
we will choose which 2 letters to be put into the correct envelops which can be done in
C(5,2) = 10 ways....further 3 letters are left which are to be dearranged such that no letter goes to the correct envelop which can be done in 3!/2! - 3!/3! = 3 - 1 = 2 ways....hence total possible cases are 10 * 2 = 20 out of 5! = 120 cases....so probability is 20/120 = 1/6