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