Question

小灯泡按红、红、绿、绿、绿、红、红、绿、绿、绿这样的顺序挂在一根绳子上，相邻灯泡相距 $6$ 英寸，并一直重复 $2$ 个红灯后接 $3$ 个绿灯的模式。第 $3$ 个红灯和第 $21$ 个红灯相距多少英尺？

注： $1$ 英尺等于 $12$ 英寸。

Small lights are hung on a string $6$ inches apart in the order red, red, green, green, green, red, red, green, green, green, and so on continuing this pattern of $2$ red lights followed by $3$ green lights. How many feet separate the $3$ rd red light and the $21$ st red light?

Note: $1$ foot is equal to $12$ inches.

$18$

$18.5$

$20$

$20.5$

$22.5$

答案：E

知识点：找规律单位换算

难度评级：1270

解答：

灯泡按每 $5$ 个一组重复，所以相邻两组的开头相距 $5\cdot6=30$ 英寸，即 $2.5$ 英尺。

每一组的开头都是一个奇数编号的红灯。第 $3$ 个红灯是第 $2$ 组的开头，第 $21$ 个红灯是第 $11$ 组的开头。

它们之间的距离是 $(11-2)\cdot2.5=22.5$ 英尺。

因此正确答案是 E。

The lights repeat in blocks of $5,$ so consecutive blocks start $5\cdot6=30$ inches, or $2.5$ feet, apart.

Each block has one odd-numbered red light beginning it. The $3$ rd red light begins the $2$ nd block and the $21$ st red light begins the $11$ th block.

The distance between them is $(11-2)\cdot2.5=22.5$ feet.

Thus, the correct answer is E.

2012 AMC 12B 第 7 题

答案：E

难度评级：1270

解答：

其他年份的第 7 题