Sum on Probability:::::::::;

1)Five different letters are put inside 5 addressed envelopes by an illeterate servent.What is the probability that only 2 letters are placed in correct envelopes?

2)An urn contains 2 white & 2 black balls .Balls are drawn succesively wihout replacement.What is the probability that a black ball appears for the 2nd time in the 4th drawing?

there are only 2W and 2B u must get 1B in the fourth draw and that must be the 2nd B so in the first 3 draws u should have got 2W and 1B in whatever order.