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2013 AMC 12B Problem 3

Problem 3 of 25EasierCombinatorics

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?

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Solution

Counting down from 201,201, the value xx is the (202−x)(202-x)th number. So 5353 is the (202−53)=149(202-53) = 149th number. Thus, the correct answer is D.
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Tagged: counting integers in a range

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