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The 19th of September 1987 was a Saturday. If 1988 was a leap-year, what day of the week was the 21st of September 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.

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 a 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 after in a week;

Thus 3 years, 1988, 1989 and 1990, will accumulate \(2+1+1=4\) days, plus 2 days of difference between 19th and 21st, which gives total of 6 days of difference (6 days shift). Thus, 21st of September 1990 was Saturday + 6 days = Friday.

I think this is a high-quality question and I agree with explanation. Though this question is inspired by a GMATPrep question the explanation is fabulous and simple!

You can solve this question in 30 seconds period. Only one simple rule to remember. "If today is a Monday, year not being a leap year, then next year same day will be a Tuesday. If Current year is a leap year then just add 2 days. So say if we are in Jan 1st 2015, Which is a Thursday, and need to find Jan 1st, 2016 just add 1 day. So Jan 1st, 2016 will be a Friday. But Jan 1st of 2017 will be Sunday since 2016 is a leap year ( we add 2 days).