M08-14

Official Solution:

On September 19th, 1987, it was a Saturday. If 1988 was a leap year, what day of the week was it on September 21st, 1990?

A. Monday
B. Tuesday
C. Wednesday
D. Thursday
E. Friday


There are 365 days in an ordinary year and 366 days in a leap year.

When 365 is divided by 7 (the number of days in a week), the remainder is 1. This means that the same date in the following ordinary year would fall one day later in the week.

When 366 is divided by 7 (the number of days in a week), the remainder is 2. This means that the same date in the following leap year would fall two days later in the week.

Considering the years 1988, 1989, and 1990, the accumulated shift in days would be \(2 + 1 + 1 = 4\) days. Additionally, there is a 2-day difference between September 19th and September 21st. This results in a total shift of 6 days. Therefore, September 21st, 1990 was 6 days after Saturday, which is a Friday.


Answer: E