Expected value: ~~ N envelope mismatch problem ~ ~

For each letter I, when the letter is put back into the original envelope, let x_i = 1. X_i = 0 when the letter is not put back in the original envelope. Then the number of letters returned to the original envelope is T = x_ 1+x_2+...+x_n but e (x _ I) = 0 * p (x _ I = 0)+1* p (x _ I =1) = 60. 2011-04-25 21:47: 27 Supplement: Actually, it seems to be influenced by other n- 1 letters, but it is not. Imagine that instead of putting the envelopes one by one, you arrange the envelopes on the ground first, and then put n letters in front of each envelope. So now you think there's something between the letters? Another example is queuing for lottery. Is it more likely that the first person will draw a certain number? Actually, it's not. In fact, whether everyone wins or not depends on others, and the chances of winning are equal.