Olympiad Maths Prep

Track / Stage 3 / 108 of 260 #108 of 2000

Problem 108

AMC 10/12, early questions
Number theory Difficulty 3.5 Find the answer

Marvin had a birthday on Tuesday, May 27 in the leap year 20082008. In what year will his birthday next fall on a Saturday?
(A) 2011(B) 2012(C) 2013(D) 2015(E) 2017\mathrm{(A)}\ 2011 \qquad \mathrm{(B)}\ 2012 \qquad \mathrm{(C)}\ 2013 \qquad \mathrm{(D)}\ 2015 \qquad \mathrm{(E)}\ 2017

Official solution

(E)\boxed{(E)} 20172017
There are 365365 days in a non-leap year. There are 77 days in a week. Since 365=527+1365 = 52 \cdot 7 + 1 (or 3651(mod7)365\equiv 1 \pmod{ 7}), the same date (after February) moves "forward" one day in the subsequent year, if that year is not a leap year.
For example:
5/27/085/27/08 Tue
5/27/095/27/09 Wed
However, a leap year has 366366 days, and 366=527+2366 = 52 \cdot 7 + 2 . So the same date (after February) moves "forward" two days in the subsequent year, if that year is a leap year.
For example:
5/27/115/27/11 Fri
5/27/125/27/12 Sun
You can keep counting forward to find that the first time this date falls on a Saturday is in 20172017:
5/27/135/27/13 Mon
5/27/145/27/14 Tue
5/27/155/27/15 Wed
5/27/165/27/16 Fri
5/27/175/27/17 Sat

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.