Mr 'X' has three children. The birthday of the first child falls on the 5th Monday of April, that of the second one falls on the 5th Thursday of November. On which day is the birthday of his third child, which falls on 20th December?

Answer is (b) November has 30 days, meaning each day of the week occurs 4 times (7 days × 4 = 28), with two days occurring 5 times. These two days will be the first two or last two days of the month. Therefore, if a day occurs 5 times, it must be at the beginning or end of the month. Since the second child's birthday falls on the fifth Thursday, it must be at the end of November. Thus, either November 29th is Thursday or November 30th is Thursday. If November 29th is Thursday, then December 20th will be a Thursday. If November 30th is Thursday, then December 20th will be a Wednesday. Since this option is not provided, the answer must be Thursday, making option (b) the correct choice.