Question

Blackwell 老师给两个班考试。上午班学生的平均分是 $84$ ，下午班学生的平均分是 $70$ 。上午班人数与下午班人数之比为 $\dfrac{3}{4}$ 。所有学生的平均分是多少？

Ms. Blackwell gives an exam to two classes. The mean of the scores of the students in the morning class is $84,$ and the afternoon class's mean score is $70.$ The ratio of the number of students in the morning class to the number of students in the afternoon class is $\dfrac{3}{4}.$ What is the mean of the scores of all the students?

$74$

$75$

$76$

$77$

$78$

答案：C

知识点：平均数比与比例

难度评级：1100

解答：

设上午班有 $3$ 名学生，下午班有 $4$ 名学生。

所有分数的总和是 $3\cdot 84+4\cdot 70$ $=252+280$ $=532$ 。

总平均数是 $\dfrac{532}{7}=76$ 。

所以正确答案是 C。

Suppose there are $3$ students in the morning class and $4$ in the afternoon class.

The total of all scores is $3\cdot 84+4\cdot 70$ $=252+280$ $=532.$

The overall mean is $\dfrac{532}{7}=76.$

Thus, the correct answer is C.

2021 AMC 12B Spring 第 4 题

答案：C

难度评级：1100

解答：

其他年份的第 4 题