2013 AMC 12B Problem 3

Attempt Problem 3 of the 2013 AMC 12B below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2013 AMC 12B solutions, or check the answer key.

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3.

When counting from 33 to 201,201, 5353 is the 5151st number counted. When counting backwards from 201201 to 3,3, 5353 is the nnth number counted. What is n?n?

146146

147147

148148

149149

150150

Answer: D
Concepts:counting integers in a range
Difficulty rating: 1100
Solution:

Counting down from 201,201, the value xx is the (202x)(202-x)th number. So 5353 is the (20253)=149(202-53) = 149th number. Thus, the correct answer is D.

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