You have n urns, the rth of which contains r-1 red balls and n-r blue balls, r=1,..., n. You pick an urn at random and remove two balls from it without replacement. Find the probability that the two...

Extracted text: You have n urns, the rth of which contains r-1 red balls and n-r blue balls, r=1,..., n. You pick an urn at random and remove two balls from it without replacement. Find the probability that the two balls are of different colours. Find the same probability when you put back a removed ball.