2. 外语
  3. How many positive integers less than 20 are equal to the sum of a positive multiple of 3 and a positive multiple of 4?

How many positive integers less than 20 are equal to the sum of a positive multiple of 3 and a positive multiple of 4?

admin2009-05-19  108
问题 How many positive integers less than 20 are equal to the sum of a positive multiple of 3 and a positive multiple of 4?
选项 A、2
B、5
C、7
D、10
E、19
答案D
解析 小于20且等于3的正倍数(positive multiple)与4的正倍数的和的正整数有多少个?
解：本题的正确答案是(D)。满足题中要求的正整数可表示为：n=3a+4b(a,b≥1)

所以满足题目要求的数共有10个。本题不一定非数出10个不可，当数出7个以上时，就可猜出10就是正确答案，因为不可能是19个。
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本试题收录于： GRE QUANTITATIVE题库GRE分类
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