Today is given as Sunday, 20th January 2017.\nExactly 90 days from today, what will be the calendar date and day of the week?

Introduction / Context:
This question combines counting days across months with basic weekday arithmetic. You are told that today is Sunday, 20 January 2017, and asked what the date and day of the week will be exactly 90 days later. Even if the given weekday for 20 January 2017 does not match the real-world calendar, you must treat the information in the question as correct and work consistently from it.

Given Data / Assumptions:

Today: 20 January 2017 (assumed to be a Sunday for the purpose of this problem).
We must move forward 90 days.
Year 2017 is a common year (February has 28 days).
Month lengths: January 31, February 28, March 31, April 30.
We calculate both the resulting date and the weekday, given the assumed starting weekday.

Concept / Approach:
First, find the calendar date that is 90 days after 20 January by counting days month by month. Then, we determine the weekday by taking 90 modulo 7, because the weekday pattern repeats every 7 days. A positive remainder tells us how many days ahead the weekday will be relative to Sunday.

Step-by-Step Solution:
Step 1: Count days remaining in January after 20 January. January has 31 days, so from 21 January to 31 January there are 11 days. We need a total of 90 days, so remaining after January = 90 - 11 = 79 days. Step 2: Subtract full months. February (28 days): remaining = 79 - 28 = 51. March (31 days): remaining = 51 - 31 = 20. So, 20 days into April after 31 March gives 20 April. Step 3: Determine the weekday after 90 days. 90 divided by 7 gives 12 full weeks and a remainder of 6 days (since 90 mod 7 = 6). Starting from Sunday, 6 days ahead: Monday (1), Tuesday (2), Wednesday (3), Thursday (4), Friday (5), Saturday (6). Therefore, 90 days after a Sunday is a Saturday.

Verification / Alternative check:
You can verify by using the ordinal day-of-year method: convert 20 January to its day-of-year number, add 90, and see that it lands on 20 April. The modulo 7 approach for weekdays gives a 6-day shift, which always moves Sunday to Saturday. This matches the option “20th April, Saturday”.

Why Other Options Are Wrong:
18th April and 19th April are 2 and 1 days earlier than the correct date, and 21st April is 1 day later, so they do not match the counted 90 days. Even if some of them have a Saturday or Sunday attached, the date plus weekday combination must both be correct, and only 20th April, Saturday satisfies that.

Common Pitfalls:
Some learners count the starting date (20 January) as day 1 instead of counting forward from the next day, which leads to an off-by-one error. Others mis-handle the month lengths and make a mistake when February or March is passed. Always subtract month by month carefully, and then use modulo 7 for the weekday shift.

Final Answer:
Exactly 90 days from Sunday, 20 January 2017, the date and day will be 20th April, Saturday.