Question

Eric 计划参加铁人三项。他在 $\tfrac14$ 英里的游泳中平均速度为每小时 $2$ 英里，在 $3$ 英里的跑步中平均速度为每小时 $6$ 英里。他的目标是在 $2$ 小时内完成比赛。为了达到目标，他在 $15$ 英里自行车赛段的平均速度必须是多少英里每小时？

Eric plans to compete in a triathlon. He can average $2$ miles per hour in the $\tfrac14$ -mile swim and $6$ miles per hour in the $3$ -mile run. His goal is to finish the triathlon in $2$ hours. To accomplish his goal what must his average speed, in miles per hour, be for the $15$ -mile bicycle ride?

$\dfrac{120}{11}$

$11$

$\dfrac{56}{5}$

$\dfrac{45}{4}$

$12$

答案：A

知识点：路程、速度与时间分数

难度评级：1030

解答：

游泳用时 $\dfrac{1/4}{2} = \dfrac18$ 小时，跑步用时 $\dfrac{3}{6} = \dfrac12$ 小时。这给自行车赛段留下 $2 - \dfrac18 - \dfrac12 = \dfrac{11}{8}$ 小时。

所需平均速度为 $\dfrac{15}{11/8} = \dfrac{120}{11}$ 英里/小时。

所以正确答案是 A。

The swim takes $\dfrac{1/4}{2} = \dfrac18$ hour and the run takes $\dfrac{3}{6} = \dfrac12$ hour. This leaves $2 - \dfrac18 - \dfrac12 = \dfrac{11}{8}$ hours for the bicycle ride.

His average speed must be $\dfrac{15}{11/8} = \dfrac{120}{11}$ miles per hour.

Thus, the correct answer is A.

2009 AMC 10A 第 4 题

答案：A

难度评级：1030

解答：

其他年份的第 4 题