Bobby's birthday in a certain year falls on Monday, 5th June. In the same year, Pinky was born on 22nd September. On which day of the week does Pinky's birthday fall in that year?

Introduction / Context:
This calendar problem, like many in aptitude tests, asks you to work out the weekday of one birthday knowing the weekday of another birthday in the same year. Bobby's birthday is given; Pinky's birthday is several months later. We must count the days between these dates and then convert this total into a shift in weekdays using the idea of modulo 7.

Given Data / Assumptions:

• Bobby's birthday: Monday, 5th June.
• Pinky's birthday: 22nd September of the same year.
• Standard month lengths:
  • June: 30 days
  • July: 31 days
  • August: 31 days
  • September: 30 days
• All dates are after February, so the relative difference is unaffected by whether the year is leap or non-leap.

Concept / Approach:
We find how many days lie between 5th June and 22nd September. Then we take this total modulo 7 to determine the net weekday shift from Monday. That shift gives us the weekday for Pinky's birthday.

Step-by-Step Solution:
Step 1: Days remaining in June after 5th June: 30 - 5 = 25 days (6th to 30th). Step 2: Full months from July to August: July: 31 days, August: 31 days. Step 3: Total days for July and August combined = 31 + 31 = 62 days. Step 4: Days in September up to 22nd September = 22 days. Step 5: Total days between 5th June and 22nd September = 25 (June) + 62 (July and August) + 22 (September) = 109 days. Step 6: Compute 109 mod 7. Since 7 * 15 = 105, remainder is 109 - 105 = 4. Step 7: A remainder of 4 means we move 4 days forward from Monday. Step 8: Starting from Monday: Tuesday (1), Wednesday (2), Thursday (3), Friday (4). Step 9: Therefore, Pinky's birthday on 22nd September falls on a Friday.

Verification / Alternative check:
Interpreting 109 days as 15 full weeks (105 days) plus 4 extra days shows that only the 4 extra days matter for weekday determination. Full weeks drop out of the calculation. From Monday, four steps forward clearly lead to Friday. This logic holds regardless of which specific year is chosen, as long as the month lengths are consistent.

Why Other Options Are Wrong:
Saturday would correspond to a 5-day shift from Monday, Wednesday to a 2-day shift, Sunday to a 6-day shift, and Monday to a 0-day shift. Since the true day difference yields a 4-day shift, none of these weekdays match the computed value, so they are incorrect.

Common Pitfalls:
Typical mistakes include mixing up the day counts for June (30 days) and the subsequent months, or mistakenly counting from 5th as day 1 rather than starting from 6th. Carefully listing each month's contribution and explicitly using modulo 7 prevents these errors and ensures a reliable answer.

Final Answer:
Pinky's birthday in that year falls on Friday.