1959 AMC 12 第 29 题

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所有题目均经美国数学协会(MAA)官方合法授权使用。

29.

一场考试共有 nn 道题,一名学生在前 2020 道题中答对了 1515 道,其余题目中答对了三分之一。每道题分值相同。若该生得分为 50%50\%,则 nn 有多少个不同的可能值?

On an examination of nn questions a student answers correctly 1515 of the first 20.20. Of the remaining questions he answers one third correctly. All the questions have the same credit. If the student’s mark is 50%,50\%, how many different values of nn can there be?

44

33

22

11

无法确定

the problem cannot be solved

答案:D
知识点:百分数一次方程整除性
难度评级:1280
小提示:

nn 表示答对的总题数

Express the total number correct in terms of nn

大提示:

15+n20315+\frac{n-20}{3} 等于 n2\frac{n}{2}

Set 15+n20315+\frac{n-20}{3} equal to n2\frac{n}{2}

解答:

得分条件给出 15+n203=n2 15+\frac{n-20}{3}=\frac n2\text{。}两边乘以 6690+2n40=3n90+2n-40=3n,所以 n=50n=50。此值也使其余题目中答对的题数为整数。因此 nn 恰有一个可能值。

因此,正确答案是 D

The score condition gives 15+n203=n2. 15+\frac{n-20}{3}=\frac n2. Multiplying by 66 yields 90+2n40=3n,90+2n-40=3n, so n=50.n=50. This value also makes the number of remaining correct answers an integer. Hence there is exactly one possible value of n.n.

Therefore, the correct answer is D.

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