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AMC12每日一题(2000年真题#18)

2018-05-07
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2000 AMC 12竞赛试题/第18题

Problem

In year $N$ , the $300^{\text{th}}$ day of the year is a Tuesday. In year $N+1$ , the $200^{\text{th}}$ day is also a Tuesday. On what day of the week did the $100$ thday of year $N-1$ occur?

$\text {(A)}\ \text{Thursday} \qquad \text {(B)}\ \text{Friday}\qquad \text {(C)}\ \text{Saturday}\qquad \text {(D)}\ \text{Sunday}\qquad \text {(E)}\ \text{Monday}$

Solution

There are either $65 + 200 = 265$ or $66 + 200 = 266$ days between the first two dates depending upon whether or not year $N$ is a leap year. Since $7$ divides into $266$ , then it is possible for both dates to be Tuesday; hence year $N$ is a leap year and $N+1$ is not a leap year. There are $265 + 300 = 565$ days between the date in years $N,N-1$ , which leaves a remainder of $5$ upon division by $7$ . Since we are subtracting days, we count 5 days before Tuesday, which gives us $\mathrm {Thursday} \text{ (A)}$ .