2018 AMC 12B Problem 10

Attempt Problem 10 of the 2018 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 2018 AMC 12B solutions, or check the answer key.

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10.

A list of 20182018 positive integers has a unique mode, which occurs exactly 1010 times. What is the least number of distinct values that can occur in the list?

202202

223223

224224

225225

234234

Answer: D
Concepts:modepigeonhole principle
Difficulty rating: 1700
Solution:

The mode uses 1010 of the entries, leaving 2008.2008. Because the mode is unique, every other value appears at most 99 times, so at least 20089=224\left\lceil\tfrac{2008}{9}\right\rceil=224 distinct non-mode values are needed.

Adding the mode gives 224+1=225.224+1=225. This is achievable: use 99 copies each of 11 through 223,223, ten copies of 224,224, and one copy of 225.225.

Thus, the correct answer is D.

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