Question

上周每次篮球训练中，Jenny 罚中球数都是前一次训练的两倍。她第五次训练罚中 $48$ 个球。她第一次训练罚中了多少个球？

At each basketball practice last week, Jenny made twice as many free throws as she made at the previous practice. At her fifth practice she made $48$ free throws. How many free throws did she make at the first practice?

$3$

$6$

$9$

$12$

$15$

答案：A

知识点：等比数列逆推法

难度评级：820

小提示：

每次训练数目都是前一次的两倍，所以倒推时每次除以二。

Each practice count is twice the previous one, so work backward by halving

大提示：

从 $48$ 开始反复减半，直到第一次训练。

Halve $48$ repeatedly to reach the first practice

解答：

从第五次训练倒推，罚中数依次为 $48, 24, 12, 6$ 和 $3$ ，对应第五、第四、第三、第二、第一次训练。

所以正确答案是 A。

Working backward from the fifth practice, the counts are $48, 24, 12, 6,$ and $3$ at the fourth, third, second, and first practices.

Thus, the correct answer is A.

2004 AMC 10B 第 3 题

答案：A

难度评级：820

小提示：

大提示：

解答：

其他年份的第 3 题