1985 AMC 8 Problem 20

Attempt Problem 20 of the 1985 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1985 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

20.

In a certain year, January had exactly four Tuesdays and four Saturdays. On what day did January 11 fall that year?
Monday
Tuesday
Wednesday
Friday
Saturday
Answer: C
Concepts:date and timemodular arithmetic
Small hint:
January has 3131 days, which is 44 full weeks plus 33 extra days
Big hint:
The three weekdays that occur five times are the days of January 1,1, 2,2, and 3;3; Tuesday and Saturday must not be among them
Solution:
Since 31=4⋅7+3,31 = 4 \cdot 7 + 3, the weekdays falling on January 1,1, 2,2, and 33 each occur five times that month, and every other weekday occurs four times.
For Tuesday and Saturday to occur only four times, neither may be January 1,1, 2,2, or 3.3. The only starting day that works is Wednesday: then Wednesday, Thursday, and Friday occur five times, while Tuesday and Saturday each occur four times.
Thus, the correct answer is C .
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