Devansh birthday in a particular non leap year falls on Friday, 14 April. In the same year, Rohan was born on 20 September. On which day of the week will Rohan birthday fall that year?

Introduction / Context:
This problem is a day of week calculation within the same calendar year. We know that 14 April is a Friday and we must determine the weekday for 20 September of that year. Such questions test your ability to count days across several months and use the concept of multiples of seven to adjust weekdays.

Given Data / Assumptions:
- Devansh birthday is Friday, 14 April. - Rohan birthday is 20 September of the same year. - The year is a normal non leap year. - Standard month lengths: April 30, May 31, June 30, July 31, August 31, September 30.

Concept / Approach:
We move forward from 14 April to 20 September, counting the number of days in between. The weekday advances by one for each day that passes. Because 7 days bring us back to the same weekday, we only need the total number of days modulo 7. Adding this remainder to the known weekday, Friday, tells us the weekday for the target date, 20 September.

Step-by-Step Solution:
Step 1: Days remaining in April after 14 April: April has 30 days, so remaining days from 15 to 30 = 16 days. Step 2: Days in May = 31 days. Step 3: Days in June = 30 days. Step 4: Days in July = 31 days. Step 5: Days in August = 31 days. Step 6: Days in September up to 20 September starting from 1 September = 20 days. Step 7: Total days from 15 April to 20 September inclusive of 20 September = 16 + 31 + 30 + 31 + 31 + 20 = 159 days. Step 8: The difference from 14 April to 20 September is 159 days. Step 9: Compute 159 mod 7. Since 7 * 22 = 154, remainder is 159 - 154 = 5. Step 10: Move 5 days forward from Friday. Friday to Saturday (1), Sunday (2), Monday (3), Tuesday (4), Wednesday (5). Step 11: Therefore, 20 September falls on Wednesday.

Verification / Alternative check:
An alternative method is to group days month wise and quickly reduce each group modulo 7. For example, 31 days in May correspond to 3 extra days beyond four weeks. Rewriting the sum as (16 + 31 + 30 + 31 + 31 + 20) and reducing each term or cumulative total modulo 7 will still lead to an overall remainder of 5. This confirms that the weekday has advanced 5 days from Friday, again giving Wednesday for 20 September.

Why Other Options Are Wrong:
- Tuesday or Thursday would correspond to a shift of 4 or 6 days from Friday, not 5. - Friday would indicate zero shift modulo 7, which contradicts the counted number of days. - Monday would match a shift of 3 days but does not align with the calculated remainder of 5.

Common Pitfalls:
Errors often come from miscounting the remaining days in April or from forgetting that some months have 30 days and others 31. Another common mistake is forgetting to use modulo 7 and trying to shift weekdays 159 times directly, which is both tedious and prone to error. Always sum the days carefully and then reduce the total modulo 7 to simplify the calculation.

Final Answer:
Rohan birthday in that year will fall on Wednesday.