AMC 8 2008 Problem 3
AMC8 2008 年第 3 题
余数与同余★★☆☆☆
If February is a month that contains Friday the , what day of the week is February 1?
如果 2 月 13 号是周五，那么 2 月 1 号是周几？
- A. / 周日
- B. / 周一
- C. / 周三
- D. / 周四
- E. / 周六
正确答案 · Correct Answer
A
解析 · Solution
Solution 1
We can go backwards by days, but we can also backwards by weeks. If we go backwards by weeks, we see that February 6 is a Friday. If we now go backwards by days, February 1 is a
Since the days of the week repeat every 7 days and , the day of the week of the day 2 days after February 13 is the same as the day of the week of February 1. Since we know that February 13 is a Friday, the day of the week of the day 2 days after February 13 is a Sunday so we get that February 1 is a .
We can go back 2 weeks and say that Friday is the -1st of February. Going forward 2 days, we get that February 1st is Sunday. So our answer is .
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