Question

学年开始时，Lisa 的目标是在全年 $50$ 次小测中至少 $80\%$ 拿到 A。前 $30$ 次小测中，她有 $22$ 次拿到 A。若她要达成目标，剩余小测中最多有多少次可以低于 A？

At the beginning of the school year, Lisa’s goal was to earn an A on at least $80\%$ of her $50$ quizzes for the year. She earned an A on $22$ of the first $30$ quizzes. If she is to achieve her goal, on at most how many of the remaining quizzes can she earn a grade lower than an A?

$1$

$2$

$3$

$4$

$5$

答案：B

知识点：百分数不等式

难度评级：960

小提示：

先求她总共需要多少个 A。

Find how many A’s she needs in total

大提示：

她需要 $0.8\cdot50$ 个 A，而还剩 $20$ 次小测。

She needs $0.8\cdot50$ A’s, and $20$ quizzes remain

解答：

Lisa 至少需要 $0.8 \cdot 50 = 40$ 次小测拿到 A。

她已经有 $22$ 个 A，所以剩下 $20$ 次中还需要 $40 - 22 = 18$ 个 A。

因此最多有 $20 - 18 = 2$ 次小测低于 A。

所以正确答案是 B。

Lisa needs an A on at least $0.8 \cdot 50 = 40$ quizzes.

She already has $22,$ so she needs $40 - 22 = 18$ A’s among the remaining $20$ quizzes.

That leaves at most $20 - 18 = 2$ quizzes with a grade lower than an A.

Thus, B is the correct answer.

2005 AMC 10B 第 6 题

答案：B

难度评级：960

小提示：

大提示：

解答：

其他年份的第 6 题