Difference between revisions of "2005 AMC 10A Problems/Problem 14"

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<math> \mathrm{(A) \ } 41\qquad \mathrm{(B) \ } 42\qquad \mathrm{(C) \ } 43\qquad \mathrm{(D) \ } 44\qquad \mathrm{(E) \ } 45 </math>
 
<math> \mathrm{(A) \ } 41\qquad \mathrm{(B) \ } 42\qquad \mathrm{(C) \ } 43\qquad \mathrm{(D) \ } 44\qquad \mathrm{(E) \ } 45 </math>
  
==Solution==
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==Solution 1==
 
If the middle digit is the average of the first and last digits, twice the middle digit must be equal to the sum of the first and last digits.  
 
If the middle digit is the average of the first and last digits, twice the middle digit must be equal to the sum of the first and last digits.  
  
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If the middle digit is <math>8</math>, possible numbers range from <math>789</math> to <math>987</math>. So there are <math>3</math> numbers in this case.  
 
If the middle digit is <math>8</math>, possible numbers range from <math>789</math> to <math>987</math>. So there are <math>3</math> numbers in this case.  
  
If the middle digit is <math>9</math>, possible numbers range from <math>999</math> to <math>999</math>. So there are <math>1</math> numbers in this case.  
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If the middle digit is <math>9</math>, the only possible number is <math>999</math>. So there is <math>1</math> number in this case.  
  
 
So the total number of three-digit numbers that satisfy the property is <math>2+4+6+8+9+7+5+3+1=45\Rightarrow E</math>
 
So the total number of three-digit numbers that satisfy the property is <math>2+4+6+8+9+7+5+3+1=45\Rightarrow E</math>
== Solution 2 (much faster and slicker)==
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Alternatively, we could note that the middle digit is uniquely defined by the first and third digits, since it is half of their sum. This also means that the sum of the first and third digits must be even. Since even numbers are formed either by adding two odd numbers or two even numbers, we can split our problem into 2 cases:
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== Solution 2 ==
 +
Alternatively, we could note that the middle digit is uniquely defined by the first and third digits since it is half of their sum. This also means that the sum of the first and third digits must be even. Since even numbers are formed either by adding two odd numbers or two even numbers, we can split our problem into 2 cases:
  
 
If both the first digit and the last digit are odd, then we have 1, 3, 5, 7, or 9 as choices for each of these digits, and there are <math>5\cdot5=25</math> numbers in this case.
 
If both the first digit and the last digit are odd, then we have 1, 3, 5, 7, or 9 as choices for each of these digits, and there are <math>5\cdot5=25</math> numbers in this case.
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The total number, then, is <math>20+25=45\Rightarrow E</math>
 
The total number, then, is <math>20+25=45\Rightarrow E</math>
  
==See Also==
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==See also==
*[[2005 AMC 10A Problems]]
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{{AMC10 box|year=2005|ab=A|num-b=13|num-a=15}}
 
 
*[[2005 AMC 10A Problems/Problem 13|Previous Problem]]
 
 
 
*[[2005 AMC 10A Problems/Problem 15|Next Problem]]
 
  
[[Category:Introductory Combinatorics Problems]]
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[[Category:Introductory Number Theory Problems]]
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{{MAA Notice}}

Revision as of 18:20, 27 December 2019

Problem

How many three-digit numbers satisfy the property that the middle digit is the average of the first and the last digits?

$\mathrm{(A) \ } 41\qquad \mathrm{(B) \ } 42\qquad \mathrm{(C) \ } 43\qquad \mathrm{(D) \ } 44\qquad \mathrm{(E) \ } 45$

Solution 1

If the middle digit is the average of the first and last digits, twice the middle digit must be equal to the sum of the first and last digits.

Doing some casework:

If the middle digit is $1$, possible numbers range from $111$ to $210$. So there are $2$ numbers in this case.

If the middle digit is $2$, possible numbers range from $123$ to $420$. So there are $4$ numbers in this case.

If the middle digit is $3$, possible numbers range from $135$ to $630$. So there are $6$ numbers in this case.

If the middle digit is $4$, possible numbers range from $147$ to $840$. So there are $8$ numbers in this case.

If the middle digit is $5$, possible numbers range from $159$ to $951$. So there are $9$ numbers in this case.

If the middle digit is $6$, possible numbers range from $369$ to $963$. So there are $7$ numbers in this case.

If the middle digit is $7$, possible numbers range from $579$ to $975$. So there are $5$ numbers in this case.

If the middle digit is $8$, possible numbers range from $789$ to $987$. So there are $3$ numbers in this case.

If the middle digit is $9$, the only possible number is $999$. So there is $1$ number in this case.

So the total number of three-digit numbers that satisfy the property is $2+4+6+8+9+7+5+3+1=45\Rightarrow E$

Solution 2

Alternatively, we could note that the middle digit is uniquely defined by the first and third digits since it is half of their sum. This also means that the sum of the first and third digits must be even. Since even numbers are formed either by adding two odd numbers or two even numbers, we can split our problem into 2 cases:

If both the first digit and the last digit are odd, then we have 1, 3, 5, 7, or 9 as choices for each of these digits, and there are $5\cdot5=25$ numbers in this case.

If both the first and last digits are even, then we have 2, 4, 6, 8 as our choices for the first digit and 0, 2, 4, 6, 8 for the third digit. There are $4\cdot5=20$ numbers here.

The total number, then, is $20+25=45\Rightarrow E$

See also

2005 AMC 10A (ProblemsAnswer KeyResources)
Preceded by
Problem 13
Followed by
Problem 15
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