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Difference between revisions of "2021 AIME I Problems/Problem 12"

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==Solution==
 
==Solution==
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The expected number of minutes depends on the distances between the frogs, in either clockwise or counterclockwise order.
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Let <math>E(a,b,c)</math> denote the expected number of minutes for two frogs to meet, such that the frogs are <math>a,b,</math> and <math>c</math> vertices apart, in either clockwise or counterclockwise order. Note that
  
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The expected number of steps depends on the distance between the frogs, not on the order in which these distances appear. Let <math>E(a,b,c)</math> where <math>a+b+c=9</math> denote the expected number of steps that it takes for two frogs to meet if traversing in clockwise or counterclockwise order, the frogs are <math>a</math>, <math>b</math> and <math>c</math> vertices apart. Then
 
  
 
<math>E(3,3,3)=1+\frac{1}{4} E(3,3,3)+\frac{3}{4} E(1,3,5)</math>, giving <math>E(3,3,3)=\frac{4}{3}+E(1,3,5)</math>; (1)
 
<math>E(3,3,3)=1+\frac{1}{4} E(3,3,3)+\frac{3}{4} E(1,3,5)</math>, giving <math>E(3,3,3)=\frac{4}{3}+E(1,3,5)</math>; (1)
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Plug in (1) and (3) into (2), we see that <math>E(1,3,5)=4</math>. <math>E(3,3,3)=\frac{4}{3}+4=\frac{16}{3}</math>. Each step is one minute. The answer is <math>16+3=\boxed{19}</math>.
 
Plug in (1) and (3) into (2), we see that <math>E(1,3,5)=4</math>. <math>E(3,3,3)=\frac{4}{3}+4=\frac{16}{3}</math>. Each step is one minute. The answer is <math>16+3=\boxed{19}</math>.
  
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-Ross Gao
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~Ross Gao (Solution)
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~MRENTHUSIASM (Reformatting and Minor Revisions)
  
 
==See Also==
 
==See Also==

Revision as of 01:57, 7 July 2021

Problem

Let $A_1A_2A_3...A_{12}$ be a dodecagon ($12$-gon). Three frogs initially sit at $A_4,A_8,$ and $A_{12}$. At the end of each minute, simultaneously, each of the three frogs jumps to one of the two vertices adjacent to its current position, chosen randomly and independently with both choices being equally likely. All three frogs stop jumping as soon as two frogs arrive at the same vertex at the same time. The expected number of minutes until the frogs stop jumping is $\frac mn$, where $m$ and $n$ are relatively prime positive integers. Find $m+n$.

Solution

The expected number of minutes depends on the distances between the frogs, in either clockwise or counterclockwise order.

Let $E(a,b,c)$ denote the expected number of minutes for two frogs to meet, such that the frogs are $a,b,$ and $c$ vertices apart, in either clockwise or counterclockwise order. Note that


$E(3,3,3)=1+\frac{1}{4} E(3,3,3)+\frac{3}{4} E(1,3,5)$, giving $E(3,3,3)=\frac{4}{3}+E(1,3,5)$; (1)

$E(1,3,5)=1+\frac{1}{8}E(1,1,7)+\frac{1}{2}E(1,3,5)+\frac{1}{8}E(3,3,3)$, giving $E(1,3,5)=2+\frac{1}{4}E(1,1,7)+\frac{1}{4}E(3,3,3)$; (2)

$E(1,1,7)=1+\frac{1}{4}E(1,1,7)+\frac{1}{4}E(1,3,5)$, giving $E(1,1,7)=\frac{4}{3}+\frac{1}{3}E(1,3,5)$; (3)

Plug in (1) and (3) into (2), we see that $E(1,3,5)=4$. $E(3,3,3)=\frac{4}{3}+4=\frac{16}{3}$. Each step is one minute. The answer is $16+3=\boxed{19}$.

~Ross Gao (Solution)

~MRENTHUSIASM (Reformatting and Minor Revisions)

See Also

2021 AIME I (Problems • Answer Key • Resources)
Preceded by
Problem 11 		Followed by
Problem 13
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15
All AIME Problems and Solutions

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