The problem says "tomorrow is Thursday".
Let's count what "4th day after tomorrow" means:
Tomorrow = Thursday
1st day after tomorrow = Friday
2nd day after tomorrow = Saturday
3rd day after tomorrow = Sunday
4th day after tomorrow = Monday
Therefore, 17th March = Monday
We need to find what day 31st March falls on.
Days between 17th and 31st:
31 - 17 = 14 \text{ days}$ <hr> Every $7$ days $=$ 1 complete week $=$ same day repeats14 \div 7 = 2 \text{ complete weeks}$
After 2 complete weeks, we land on the same day.
Let's verify:
17th March = Monday
17+7=24th March = Monday
24+7=31st March = Monday
31st March falls on Monday