Question

Fort Worth Zoo 有一些两条腿的鸟和一些四条腿的哺乳动物。Margie 某次参观动物园时数到二百个头和五百二十二条腿。Margie 数到的动物中有多少只是两条腿的鸟？

The Fort Worth Zoo has a number of two-legged birds and a number of four-legged mammals. On one visit to the zoo, Margie counted 200 heads and 522 legs. How many of the animals that Margie counted were two-legged birds?

$61$

$122$

$139$

$150$

$161$

答案：C

知识点：方程组

难度评级：1100

视频讲解：

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文字解答：

设 $f$ 为 $4$ 条腿动物的数量， $t$ 为 $2$ 条腿动物的数量。

数腿得到 $2t+4f = 522$ ，数头得到 $t + f = 200$ 。将第二式乘以四，得 $4t+4f = 800$ 。减去第一式，得到 $2t = 278 \implies t = 139$ 。因此有 $139$ 只两条腿的鸟。

所以正确答案是 C。

Let $f$ be the number of animals with $4$ legs and let $t$ be the number of animals with $2$ legs.

Counting the number of legs yields $2t+4f = 522$ and counting the number of heads yields $t + f = 200.$ This means $4t+4f = 800,$ and subtracting the first equation from the second yields $2t = 278 \implies t = 139.$ This means there are $139$ two-legged birds.

Thus, the answer is C .

2012 AMC 8 第 9 题

答案：C

难度评级：1100

视频讲解：

文字解答：

其他年份的第 9 题