2021 CMC 12A Problems/Problem 4

The following problem is from both the 2021 CMC 12A #4 and 2021 CMC 10A #5, so both problems redirect to this page.

Problem

There exists a positive integer $N$ such that $\frac{\tfrac{1}{999}+\tfrac{1}{1001}}{2}=\frac{1}{N}.$ What is the sum of the digits of $N$ ?

$\textbf{(A) } 36\qquad\textbf{(B) } 45\qquad\textbf{(C) } 54\qquad\textbf{(D) } 63\qquad\textbf{(E) } 72\qquad$

Solution

Let $a=1000$ . Then $\frac{\tfrac{1}{a-1}+\frac{1}{a+1}}{2}-\frac{1}{a}=\frac{1}{N}$ or $\frac{1}{a^{3}-a}=\frac{1}{N}$ , so $N=a^{3}-a=1000^{3}-1000=999,999,000$ whose digits sum to $\boxed{\textbf{(C) } 54}$ .

Video Solution

https://youtu.be/H4uIpLB-6Jo

~Punxsutawney Phil

2021 CMC 12A (Problems • Answer Key • Resources)
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2021 CMC 10A (Problems • Answer Key • Resources)
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2021 CMC 12A Problems/Problem 4

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Video Solution

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