2000 AMC 10 Problem 25

Attempt Problem 25 of the 2000 AMC 10 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 10 solutions, or check the answer key.

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25.

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: 1860
Small Hint:

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

Big Hint:

Count the days from day 300300 of year NN to day 200200 of year N+1N+1 to decide whether NN is a leap year

Solution:

From day 300300 of year NN to day 200200 of year N+1N + 1 is (L300)+200(L - 300) + 200 days, where LL is the length of year N.N. If NN were not a leap year, this is 2656(mod7),265 \equiv 6 \pmod 7, giving a Monday, not a Tuesday. So year NN is a leap year, and the count is 266=738,266 = 7 \cdot 38, consistent with Tuesday.

Then years N1N - 1 and N+1N + 1 are not leap years.

The 100100th day of year N1N - 1 precedes the Tuesday (day 300300 of year NN) by (365100)+300=565(365 - 100) + 300 = 565 days. Since 565=780+5,565 = 7 \cdot 80 + 5, that day is 55 days earlier in the week than Tuesday, which is a Thursday.

Thus, the correct answer is A.

Problem 24#24
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