Twenty pairs of individuals playing in a bridge tournament have been seeded 1, 20. in the first part of the tournament , the 20 are randomly divided into 10 east-west pairs and 10 north -south pairs.
a. what is the probability that x of the top pairs end up playing east-west?
b. what is the probability that all of the top five pairs end up playing the same direction? c. if there are 2n pairs , what is the pmf of x=the number among the top n pairs who end up playing east-west? what are e(x) and v(x)?