AMC12 2013 B

AMC12 2013 B · Q3

AMC12 2013 B · Q3. It mainly tests Arithmetic sequences basics.

When counting from 3 to 201, 53 is the 51st number counted. When counting backwards from 201 to 3, 53 is the $n$th number counted. What is $n$?

从3数到201时，53是第51个数。从201反向数到3时，53是第$n$个数。$n$是多少？

(A) 146 146

(B) 147 147

(D) 149 149

(E) 150 150

Answer

Correct choice: (D)

正确答案：（D）

Solution

The number 201 is the 1st number counted when proceeding backwards from 201 to 3. In turn, 200 is the 2nd number, 199 is the 3rd number, and $x$ is the $(202 - x)$th number. Therefore 53 is the $(202 - 53)$th number, which is the 149th number.

从201到3反向数时，201是第1个数，200是第2个数，199是第3个数，$x$是第$(202 - x)$个数。因此53是第$(202 - 53)$个数，即第149个数。

Topics

AR.012 Arithmetic sequences basics