Difference between revisions of "2019 AMC 10B Problems/Problem 25"

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<math>\textbf{(A) }55\qquad\textbf{(B) }60\qquad\textbf{(C) }65\qquad\textbf{(D) }70\qquad\textbf{(E) }75</math>
 
<math>\textbf{(A) }55\qquad\textbf{(B) }60\qquad\textbf{(C) }65\qquad\textbf{(D) }70\qquad\textbf{(E) }75</math>
  
==Solution 1 (Recursion)==
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==Solution 1 (recursion)==
We can deduce that any valid sequence of length <math>n</math> will start with a 0 followed by either "10" or "110".
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We can deduce, from the given restrictions, that any valid sequence of length <math>n</math> will start with a <math>0</math> followed by either <math>10</math> or <math>110</math>.
Because of this, we can define a recursive function:
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Thus we can define a recursive function <math>f(n) = f(n-3) + f(n-2)</math>, where <math>f(n)</math> is the number of valid sequences of length <math>n</math>.
  
<math>f(n) = f(n-3) + f(n-2)</math>
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This is because for any valid sequence of length <math>n</math>, you can append either <math>10</math> or <math>110</math> and the resulting sequence will still satisfy the given conditions.
  
This is because for any valid sequence of length <math>n</math>, you can append either "1010" or "0110" and the resulting sequence would still satisfy the given conditions.
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It is easy to find <math>f(5) = 1</math> and <math>f(6) = 2</math> by hand, and then by the recursive formula, we have <math>f(19) = \boxed{\textbf{(C) }65}</math>.
  
<math>f(5) = 1</math> and <math>f(6) = 2</math>, so you follow the recursion up until <math>f(19) = 65 \quad \boxed{C}</math>
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==Solution 2 (casework)==
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After any particular <math>0</math>, the next <math>0</math> in the sequence must appear exactly <math>2</math> or <math>3</math> positions down the line. In this case, we start at position <math>1</math> and end at position <math>19</math>, i.e. we move a total of <math>18</math> positions down the line. Therefore, we must add a series of <math>2</math>s and <math>3</math>s to get <math>18</math>. There are a number of ways to do this:
  
==Solution 2 (Casework)==
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'''Case 1''': nine <math>2</math>s - there is only <math>1</math> way to arrange them.
After any given zero, the next zero must appear exactly two or three spots down the line. And we started at position 1 and ended at position 19, so we moved over 18. Therefore, we must add a series of 2's and 3's to get 18. How can we do this?
 
  
Option 1: nine 2's (there is only 1 way to arrange this).
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'''Case 2''': two <math>3</math>s and six <math>2</math>s - there are <math>{8\choose2} = 28</math> ways to arrange them.
  
Option 2: two 3's and six 2's (<math>{8\choose2} =28</math> ways to arrange this).
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'''Case 3''': four <math>3</math>s and three <math>2</math>s - there are <math>{7\choose4} = 35</math> ways to arrange them.
  
Option 3: four 3's and three 2's (<math>{7\choose3}=35</math> ways to arrange this).
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'''Case 4''': six <math>3</math>s - there is only <math>1</math> way to arrange them.
  
Option 4: six 3's (there is only 1 way to arrange this).
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Summing the four cases gives <math>1+28+35+1=\boxed{\textbf{(C) }65}</math>.
 
 
Sum the four numbers given above: 1+28+35+1=65
 
  
 
==Video Solution==
 
==Video Solution==
 
For those who want a video solution: https://youtu.be/VamT49PjmdI
 
For those who want a video solution: https://youtu.be/VamT49PjmdI
 +
 
==See Also==
 
==See Also==
 
{{AMC10 box|year=2019|ab=B|num-b=24|after=Last Problem}}
 
{{AMC10 box|year=2019|ab=B|num-b=24|after=Last Problem}}
 
{{AMC12 box|year=2019|ab=B|num-b=22|num-a=24}}
 
{{AMC12 box|year=2019|ab=B|num-b=22|num-a=24}}
 
{{MAA Notice}}
 
{{MAA Notice}}
SUB2PEWDS
 

Revision as of 20:32, 26 September 2019

The following problem is from both the 2019 AMC 10B #25 and 2019 AMC 12B #23, so both problems redirect to this page.

Problem

How many sequences of $0$s and $1$s of length $19$ are there that begin with a $0$, end with a $0$, contain no two consecutive $0$s, and contain no three consecutive $1$s?

$\textbf{(A) }55\qquad\textbf{(B) }60\qquad\textbf{(C) }65\qquad\textbf{(D) }70\qquad\textbf{(E) }75$

Solution 1 (recursion)

We can deduce, from the given restrictions, that any valid sequence of length $n$ will start with a $0$ followed by either $10$ or $110$. Thus we can define a recursive function $f(n) = f(n-3) + f(n-2)$, where $f(n)$ is the number of valid sequences of length $n$.

This is because for any valid sequence of length $n$, you can append either $10$ or $110$ and the resulting sequence will still satisfy the given conditions.

It is easy to find $f(5) = 1$ and $f(6) = 2$ by hand, and then by the recursive formula, we have $f(19) = \boxed{\textbf{(C) }65}$.

Solution 2 (casework)

After any particular $0$, the next $0$ in the sequence must appear exactly $2$ or $3$ positions down the line. In this case, we start at position $1$ and end at position $19$, i.e. we move a total of $18$ positions down the line. Therefore, we must add a series of $2$s and $3$s to get $18$. There are a number of ways to do this:

Case 1: nine $2$s - there is only $1$ way to arrange them.

Case 2: two $3$s and six $2$s - there are ${8\choose2} = 28$ ways to arrange them.

Case 3: four $3$s and three $2$s - there are ${7\choose4} = 35$ ways to arrange them.

Case 4: six $3$s - there is only $1$ way to arrange them.

Summing the four cases gives $1+28+35+1=\boxed{\textbf{(C) }65}$.

Video Solution

For those who want a video solution: https://youtu.be/VamT49PjmdI

See Also

2019 AMC 10B (ProblemsAnswer KeyResources)
Preceded by
Problem 24
Followed by
Last Problem
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 10 Problems and Solutions
2019 AMC 12B (ProblemsAnswer KeyResources)
Preceded by
Problem 22
Followed by
Problem 24
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
All AMC 12 Problems and Solutions

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