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Difference between revisions of "1984 AIME Problems/Problem 4"
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== Problem ==
 
== Problem ==
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Let <math>\displaystyle S</math> be a list of positive integers - not necessarily distinct - in which the number <math>\displaystyle 68</math> appears. The arithmetic mean of the numbers in <math>\displaystyle S</math> is <math>\displaystyle 56</math>. However, if <math>\displaystyle 68</math> is removed, the arithmetic mean of the numbers is <math>\displaystyle 55</math>. What's the largest number that can appear in <math>\displaystyle S</math>?
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Let <math>\displaystyle S</math> be a list of [[positive integer]]s - not necessarily [[distinct]] - in which the number <math>\displaystyle 68</math> appears. The [[arithmetic mean]] of the numbers in <math>\displaystyle S</math> is <math>\displaystyle 56</math>. However, if <math>\displaystyle 68</math> is removed, the arithmetic mean of the numbers is <math>\displaystyle 55</math>. What's the largest number that can appear in <math>\displaystyle S</math>?
  
 
== Solution ==
 
== Solution ==
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{{solution}}
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Suppose <math>S</math> has <math>n</math> members other than 68, and the sum of these members is <math>s</math>.  Then we're given that <math>\frac{s + 68}{n + 1} = 56</math> and <math>\frac{s}{n} = 55</math>.  Multiplying to clear [[denominator]]s, we have <math>s + 68 = 56n + 56</math> and <math>s = 55n</math> so <math>68 = n + 56</math>, <math>n = 12</math> and <math>s = 12\cdot 55 = 660</math>.  Because the sum and number of the elements of <math>S</math> are fixed, if we want to maximize the largest number in <math>S</math>, we should take all but one member of <math>S</math> to be as small as possible.  Since all members of <math>S</math> are positive integers, the smallest possible value of a member is 1.  Thus the largest possible element is <math>660 - 11 = 649</math>.
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== See also ==
 
== See also ==
 
* [[1984 AIME Problems/Problem 3 | Previous problem]]
 
* [[1984 AIME Problems/Problem 3 | Previous problem]]
 
* [[1984 AIME Problems/Problem 5 | Next problem]]
 
* [[1984 AIME Problems/Problem 5 | Next problem]]
 
* [[1984 AIME Problems]]
 
* [[1984 AIME Problems]]
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[[Category:Intermediate Algebra Problems]]

Revision as of 12:18, 24 January 2007

Problem

Let $\displaystyle S$ be a list of positive integers - not necessarily distinct - in which the number $\displaystyle 68$ appears. The arithmetic mean of the numbers in $\displaystyle S$ is $\displaystyle 56$. However, if $\displaystyle 68$ is removed, the arithmetic mean of the numbers is $\displaystyle 55$. What's the largest number that can appear in $\displaystyle S$?

Solution

Suppose $S$ has $n$ members other than 68, and the sum of these members is $s$. Then we're given that $\frac{s + 68}{n + 1} = 56$ and $\frac{s}{n} = 55$. Multiplying to clear denominators, we have $s + 68 = 56n + 56$ and $s = 55n$ so $68 = n + 56$, $n = 12$ and $s = 12\cdot 55 = 660$. Because the sum and number of the elements of $S$ are fixed, if we want to maximize the largest number in $S$, we should take all but one member of $S$ to be as small as possible. Since all members of $S$ are positive integers, the smallest possible value of a member is 1. Thus the largest possible element is $660 - 11 = 649$.

See also

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