2006 AMC 10B Problems/Problem 16

Problem

Leap Day, February 29, 2004, occured on a Sunday. On what day of the week will Leap Day, February 29, 2020, occur?

$\mathrm{(A) \ } Tuesday \qquad \mathrm{(B) \ } Wednesday \qquad \mathrm{(C) \ } Thursday \qquad \mathrm{(D) \ } Friday \qquad \mathrm{(E) \ } Saturday$

Solution

There are $365$ days in a year, plus $1$ extra day if there is a Leap Day, which occurs on years that are multiples of 4 (with a few exceptions that don't affect this problem).

Therefore, the number of days between Leap Day 2004 and Leap Day 2020 is:

$16 \cdot 365 + 4 \cdot 1 = 5844$

Since the days of the week repeat every $7$ days and $5844 \equiv -1 \bmod{7}$ , the day of the week Leap Day 2020 occurs is the day of the week the day before Leap Day 2004 occurs which is $Saturday \Rightarrow E$

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2006 AMC 10B Problems/Problem 16

Problem

Solution

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