Forty ladies and fifty gentlemen are present at a party.

Since there are 20 couples, 20 of the 40 ladies are married and the other 20 are single. Similarly, 20 of the 50 gentlemen are married and the other 30 are single. The probability of selecting a married lady is 1/2 (20 out of 40), but once the lady is picked, there is only a 1 in 50 chance of picking a man who is her husband. Thus, the probability of picking a man who is her husband is 1/50 and the probability of picking a married couple is 1/2 x 1/50 = 1/100.

Alternative solution:

We’ve approached the problem from the angle of the lady. We could have approached it from the angle of the man also.

The probability of selecting a married man is 2/5 (20 out of 50), but once the man is picked, there is only a 1 in 40 chance of picking a lady who is his wife. Thus, the probability of picking a lady who is his wife is 1/40 and the probability of picking a married couple is 2/5 x 1/40 = 2/200 = 1/100.

Answer: B