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lcq0915 · 发表于 2012-3-25 20:47:01

第三百零七题

（题目一摸一样）When positive integer x is divided by 5,the remainder is 3; and when x is divided by 7, the remainder is 4. Whenpositive integer y is divided by 5, the remainder is 3; and when y is dividedby 7, the remainder is 4. If x > y, which of the following must be a factorof x - y?

A.12B.15 C.20 D.28 E.35
答案 E
硬代了数字进去算，可以得出答案，但是希望知道具体做法！求牛牛们解答啊~

treewing929 · 发表于 2012-3-25 20:51:49

第三百零七题

（题目一摸一样）When positive integer x is divided by 5,the remainder is 3; and when x is divided by 7, the remainder is 4. Whenpositive integer y is divided by 5, the remainder is 3; and when y is dividedby 7, the remainder is 4. If x > y, which of the following must be a factorof x - y?

A.12B.15 C.20 D.28 E.35
答案 E
硬代了数字进去算，可以得出答案，但是希望知道具体做法！求牛牛们解答啊~

-- by 会员 lcq0915 (2012/3/25 20:47:01)

X=5A+3, Y=5C+3, 所以, X-Y=5(A-C)
X=7B+4, Y=7D+4, 所以, X-Y=7(B-D)
綜合, X-Y 是 5 和 7 公倍數. 所以 35

lcq0915 · 发表于 2012-3-26 14:00:14

多谢多谢

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