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Difference between revisions of "2012 AIME II Problems/Problem 12"

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This means that by the Chinese Remainder Theorem, <math>n</math> can have <math>2\cdot 6 \cdot 8 = 96</math> different residues mod <math>7 \cdot 11 \cdot 13 = 1001</math>. Thus, there are <math>960</math> values of <math>n</math> satisfying the conditions in the range <math>0 \le n < 10010</math>. However, we must now remove any values greater than <math>10000</math> that satisfy the conditions. By checking residues, we easily see that the only such values are <math>10006</math> and <math>10007</math>, so there remain <math>\fbox{958}</math> values satisfying the conditions of the problem.
 
This means that by the Chinese Remainder Theorem, <math>n</math> can have <math>2\cdot 6 \cdot 8 = 96</math> different residues mod <math>7 \cdot 11 \cdot 13 = 1001</math>. Thus, there are <math>960</math> values of <math>n</math> satisfying the conditions in the range <math>0 \le n < 10010</math>. However, we must now remove any values greater than <math>10000</math> that satisfy the conditions. By checking residues, we easily see that the only such values are <math>10006</math> and <math>10007</math>, so there remain <math>\fbox{958}</math> values satisfying the conditions of the problem.
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=== Solution 2 (Probability)===
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Note that <math>\operatorname{lcm}(7, 11, 13)=1001</math> and <math>10000=1001\cdot10-10</math>. Now, consider any positive integer <math>n</math> such that <math>1\le n\le 10010</math>. Since all <math>7</math>-safe numbers are of the form <math>7k+3</math> or <math>7k+4</math>, the probability that <math>n</math> is <math>7</math>-safe is <math>\frac{2}{7}</math>. Similarly, the probability that <math>n</math> is <math>11</math>-safe is <math>\frac{6}{11}</math> and the probability that <math>n</math> is <math>13</math>-safe is <math>\frac{8}{13}</math>. Furthermore, since <math>7, 11, \text{ and } 13</math> are all relatively prime, <math>n</math> being <math>7</math>-safe, <math>11</math>-safe, and <math>13</math>-safe are all independent events. Thus, the number of positive integers less then or equal to <math>10010</math> which are simultaneously <math>7</math>-safe, <math>11</math>-safe, and <math>13</math>-safe is <math>10010\left(\frac{2}{7}\right)\left(\frac{6}{11}\right)\left(\frac{8}{13}\right)=960</math>. However, the problem asks for numbers less than or equal to <math>10000</math>, so we must subtract any numbers we counted which are greater than <math>10000</math>. This is easy; we can see that <math>10006</math> and <math>10007</math> were counted, so the answer is <math>960-2=\boxed{958}</math>.
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-brainiacmaniac31
  
 
== See Also ==
 
== See Also ==
 
{{AIME box|year=2012|n=II|num-b=11|num-a=13}}
 
{{AIME box|year=2012|n=II|num-b=11|num-a=13}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Revision as of 15:19, 3 June 2020

Problem 12

For a positive integer $p$, define the positive integer $n$ to be $p$-safe if $n$ differs in absolute value by more than $2$ from all multiples of $p$. For example, the set of $10$-safe numbers is $\{ 3, 4, 5, 6, 7, 13, 14, 15, 16, 17, 23, \ldots\}$. Find the number of positive integers less than or equal to $10,000$ which are simultaneously $7$-safe, $11$-safe, and $13$-safe.

Solution

We see that a number $n$ is $p$-safe if and only if the residue of $n \mod p$ is greater than $2$ and less than $p-2$; thus, there are $p-5$ residues $\mod p$ that a $p$-safe number can have. Therefore, a number $n$ satisfying the conditions of the problem can have $2$ different residues $\mod 7$, $6$ different residues $\mod 11$, and $8$ different residues $\mod 13$. The Chinese Remainder Theorem states that for a number $x$ that is $a$ (mod b) $c$ (mod d) $e$ (mod f) has one solution if $gcd(b,d,f)=1$. For example, in our case, the number $n$ can be: 3 (mod 7) 3 (mod 11) 7 (mod 13) so since $gcd(7,11,13)$=1, there is 1 solution for n for this case of residues of $n$.

This means that by the Chinese Remainder Theorem, $n$ can have $2\cdot 6 \cdot 8 = 96$ different residues mod $7 \cdot 11 \cdot 13 = 1001$. Thus, there are $960$ values of $n$ satisfying the conditions in the range $0 \le n < 10010$. However, we must now remove any values greater than $10000$ that satisfy the conditions. By checking residues, we easily see that the only such values are $10006$ and $10007$, so there remain $\fbox{958}$ values satisfying the conditions of the problem.


Solution 2 (Probability)

Note that $\operatorname{lcm}(7, 11, 13)=1001$ and $10000=1001\cdot10-10$. Now, consider any positive integer $n$ such that $1\le n\le 10010$. Since all $7$-safe numbers are of the form $7k+3$ or $7k+4$, the probability that $n$ is $7$-safe is $\frac{2}{7}$. Similarly, the probability that $n$ is $11$-safe is $\frac{6}{11}$ and the probability that $n$ is $13$-safe is $\frac{8}{13}$. Furthermore, since $7, 11, \text{ and } 13$ are all relatively prime, $n$ being $7$-safe, $11$-safe, and $13$-safe are all independent events. Thus, the number of positive integers less then or equal to $10010$ which are simultaneously $7$-safe, $11$-safe, and $13$-safe is $10010\left(\frac{2}{7}\right)\left(\frac{6}{11}\right)\left(\frac{8}{13}\right)=960$. However, the problem asks for numbers less than or equal to $10000$, so we must subtract any numbers we counted which are greater than $10000$. This is easy; we can see that $10006$ and $10007$ were counted, so the answer is $960-2=\boxed{958}$. -brainiacmaniac31

See Also

2012 AIME II (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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