Ten Married couples.
Let Xj indicate if the jth man sits across from his wife. Then E(Xj) = 1/19,
since no matter where the man sits, his wife can sit in 19 other possible chairs,
only 1 of which is directly across from him.
Thus the expected number of men sitting across from their wives is :
E(X1 + .... + X10) = E(X1) + ... + E(X10) = 1/19 + ..... + 1/19 = 10/19=0,53."
