In a certain year, Nasir has a birthday on Thursday, 18 May. In the same calendar year, Rehan was born on 19 August. On which day of the week does Rehan's birthday fall in that year?

Introduction / Context:

This reasoning question checks understanding of calendars and day counting within a single year. We are given the day of the week for one specific date and must find the day for another date later in the same year. It is a classic exam problem that tests careful counting of days and working with remainders modulo seven.

Given Data / Assumptions:

• Nasir has a birthday on Thursday, 18 May of a particular year.
• Rehan has a birthday on 19 August of the same year.
• We assume a normal Gregorian calendar with May having 31 days, June 30 days, July 31 days and August 31 days.
• We need only the difference in days between the two dates within the same year, so leap year status does not affect the answer.

Concept / Approach:

The day of the week advances by one for each extra day. Therefore, if the second date is n days after the first date, we simply add n (modulo 7) to the known starting day. To apply this, we compute the exact number of days between 18 May and 19 August and then use the remainder when divided by seven to shift from Thursday to the required day.

Step-by-Step Solution:

Verification / Alternative check:

We can verify by grouping days into complete weeks. Ninety three days consist of 13 full weeks (91 days) plus 2 extra days. Full weeks do not change the day of the week, so only the extra 2 days matter, again confirming that the weekday moves from Thursday to Saturday.

Why Other Options Are Wrong:

Wednesday would correspond to moving backward rather than forward, which contradicts the fact that August is after May.

Friday would be correct only if the difference were 1 day modulo seven, which is not true.

Thursday would require the difference to be a multiple of seven days with no remainder, which is also not the case.

Common Pitfalls:

A common mistake is to count both starting and ending dates or to miscount days in a month. Another error is forgetting to use modulo seven and instead using the raw day difference. Writing each month's contribution separately and then using the remainder eliminates such errors in calendar questions.

Final Answer:

Rehan's birthday in that year falls on Saturday.