2000 AMC 12 Problem 18

Attempt Problem 18 of the 2000 AMC 12 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 2000 AMC 12 solutions, or check the answer key.

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

18.

In year N,N, the 300300th day of the year is a Tuesday. In year N+1,N + 1, the 200200th day is also a Tuesday. On what day of the week did the 100100th day of year N1N - 1 occur?

Thursday

Friday

Saturday

Sunday

Monday

Answer: A
Concepts:date and timemodular arithmetic
Difficulty rating: 1870
Small Hint:

Two dates fall on the same weekday exactly when the number of days between them is a multiple of 77

Big Hint:

Use the two given Tuesdays to decide which of N1,N,N+1N - 1, N, N + 1 is a leap year

Solution:

From day 300300 of year NN to day 200200 of year N+1,N + 1, the number of days is 365300+200=265365 - 300 + 200 = 265 if NN is not a leap year. But 265=737+6,265 = 7 \cdot 37 + 6, which would land on a Monday, not a Tuesday.

So year NN is a leap year, and the gap is 266=738266 = 7 \cdot 38 days, giving a Tuesday as stated. It follows that year N1N - 1 is not a leap year.

The 100100th day of year N1N - 1 precedes the Tuesday on day 300300 of year NN by 365100+300=565365 - 100 + 300 = 565 days. Since 565=780+5,565 = 7 \cdot 80 + 5, that day is 55 weekdays before Tuesday, namely Thursday.

Thus, the correct answer is A.

Problem 17#17
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