2014 AMC 10A Problems/Problem 25

The following problem is from both the 2014 AMC 12A #22 and 2014 AMC 10A #25, so both problems redirect to this page.

Problem

The number $5^{867}$ is between $2^{2013}$ and $2^{2014}$ . How many pairs of integers $(m,n)$ are there such that $1\leq m\leq 2012$ and $5^n<2^m<2^{m+2}<5^{n+1}?$ $\textbf{(A) }278\qquad \textbf{(B) }279\qquad \textbf{(C) }280\qquad \textbf{(D) }281\qquad \textbf{(E) }282\qquad$

Solution

Between any two consecutive powers of 5 there are either 2 or 3 powers of 2 (because $2^2<5^1<2^3$ ). Consider the intervals $(5^0,5^1),(5^1,5^2),\dots (5^{866},5^{867})$ . We want the number of intervals with 3 powers of 2.

From the given that $2^{2013}<5^{867}<2^{2014}$ , we know that these 867 intervals together have 2013 powers of 2. Let $x$ of them have 2 powers of 2 and $y$ of them have 3 powers of 2. Thus we have the system

\[x+y&=867\] (Error compiling LaTeX. Unknown error_msg)

\[2x+3y&=2013\] (Error compiling LaTeX. Unknown error_msg)

from which we get $y=279$ , so the answer is $\boxed{\textbf{(B)}}$ .

(Solution by superpi83)

2014 AMC 10A (Problems • Answer Key • Resources)
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2014 AMC 12A (Problems • Answer Key • Resources)
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2014 AMC 10A Problems/Problem 25

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