1 January 2001 was a Monday. On which day of the week will 1 January 2101 fall?

SundayCorrect answerYour answerSaturdayCorrect answerYour answerMondayCorrect answerYour answerFridayCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer B

Correct answer: B

The years 2001 to 2100 must be counted. Leap years among them: 2004, 2008, …, 2096, which is 24 years. 2100 is NOT leap (a century year not divisible by 400).
Odd days $= 24 \times 2 + 76 \times 1 = 124$ .
$124 = 17 \times 7 + 5$ , so 5 odd days.
Monday moved on by 5 days is Saturday.

Worth noting

Sunday comes from treating 2100 as a leap year (25 leap years, 6 odd days). Monday assumes 100 years always give 0 odd days; in fact 100 years give 5.

Concept: Century rule for leap years in an odd-days count
Typical timeTypical: 120 s
Your timeYours: —

The rule this testsDay of the week

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Ten questions from Calendars and Clocks, at your own pace.

1 January 2001 was a Monday. On which day of the week will 1 January 2101 fall?

Explanation