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Three letters are dictated to three persons and an envelope is addressed to each of them, the letters are inserted into the envelopes at random so that each envelope contains exactly one letter.

6.  Three letters are dictated to three persons and an envelope is addressed to each of them, the letters are inserted into the envelopes at random so that each envelope contains exactly one letter. Find the probability that at least one letter is in its proper envelope.

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Given, 3 letters are put in 3 envelopes.

The number of ways of putting the 3 different letters randomly = 3!

Number of ways that at least one of the 3 letters is in the correct envelope 

 = No. of ways that exactly 1 letter is in correct envelope +  No. of ways that 2 letters are in the correct envelope(The third is automatically placed correctly)

= No. of ways that exactly 1 letter is in correct envelope +  No. of ways that all the 3 letters are in the correct envelope

(^{3}\textrm{C}_{1}\times1) + 1 = 4

(Explanation for ^{3}\textrm{C}_{1}\times1 :

No. of ways of selecting 1 envelope out of 3 = ^{3}\textrm{C}_{1}.

If we put the correct letter in it, there is only one way the other two are put in the wrong envelope! )

Therefore, the probability that at least one letter is in its proper envelope = \frac{4}{3!} = \frac{4}{6} = \frac{2}{3}

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