Probability basics

10 letters are placed in 10 envelopes. A child finds 5 enveloped letters and interchanges them such that none of them is in correct place. Then all the 10 letters are arranged in some random order. One person picks one of them. Find the probability that the letter is correct enveloped. Let the answer be A. and the no. of ways the child can do so be B. Then find A*B.

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