M24-10

Official Solution:

The 19th of September 1987 was Saturday. What day was the 20th of September 1990 if 1988 was a leap-year?

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.

365 divided by 7 (a week period) yields a remainder of 1, thus the same date after an ordinary year would fall on the next day of the week;

366 divided by 7 (a week period) yields a remainder of 2, thus the same date after a leap year would fall two days later in the week;

Thus 3 years, 1988, 1989, and 1990, will accumulate \(2+1+1=4\) days, plus the 1-day difference between the 19th and 20th, which gives a total of a 5-day difference (5-day shift). Thus, the 20th of September 1990 was \(Saturday \ + \ 5 \ days = Thursday\).


Answer: D