Hi everybody. Once more, I need your help. The following is a problem that appears in Rohatgi's "An introduction to Probability and Statistics": "Each of n urns contains four white and six black balls, while another urn contains five white and five black balls. An urn is chosen at random from the (n+1) urns, and two balls are drawn from it, both being black. The probability that five white and three black balls remain in the chosen urn is 1/7. Find n." My first guess would be that n=6, but the solution given in the book is n=4. Someone can tell me if I am wrong or there is a mistake in the book ? Thanks.