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Difference between revisions of "2000 AMC 12 Problems/Problem 6"

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Note: once we apply the factoring trick we see that, since <math>p-1</math> and <math>q-1</math> are even, <math>K+1</math> should be a multiple of <math>4</math>.  
 
Note: once we apply the factoring trick we see that, since <math>p-1</math> and <math>q-1</math> are even, <math>K+1</math> should be a multiple of <math>4</math>.  
  
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These means that only <math> 119 \Rightarrow C </math> and <math>231 \rightarrow E</math> are possible.
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These means that only <math> 119 \Rightarrow C </math> and <math>231 \Rightarrow E</math> are possible.
  
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We can't have <math>(p-1) \cdot (q-1)=232=2^3\cdot 29</math> with <math>p</math> and <math>q</math> below <math>18</math> (why?).
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We can't have <math>(p-1) \cdot (q-1)=232=2^3\cdot 29</math> with <math>p</math> and <math>q</math> below <math>18</math>. Indeed, <math>(p-1) \cdot (q-1)</math> would have to be <math>2 \cdot 116</math> or <math>4 \cdot 58</math>.
  
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But <math>(p-1) \cdot (q-1)=120=2^3\cdot 3 \cdot 5</math> could be <math>2 \cdot 60,4 \cdot 30,6 \cdot 20</math> or <math>10 \cdot 12.</math> Of these, three have <math>p</math> and <math>q</math> prime but only the last has them both small enough.
+
But <math>(p-1) \cdot (q-1)=120=2^3\cdot 3 \cdot 5</math> could be <math>2 \cdot 60,4 \cdot 30,6 \cdot 20</math> or <math>10 \cdot 12.</math> Of these, three have <math>p</math> and <math>q</math> prime, but only the last has them both small enough.
  
 
== See also ==
 
== See also ==

Revision as of 02:39, 22 July 2008

Problem

Two different prime numbers between $4$ and $18$ are chosen. When their sum is subtracted from their product, which of the following numbers could be obtained?

$\mathrm{(A) \ 21 } \qquad \mathrm{(B) \ 60 } \qquad \mathrm{(C) \ 119 } \qquad \mathrm{(D) \ 180 } \qquad \mathrm{(E) \ 231 }$

Solution

Let the primes be $p$ and $q$.

The problem asks us for possible values of $K$ where $K=pq-p-q$

Using Simon's Favorite Factoring Trick:

$K+1=pq-p-q+1$

$K+1=(p-1)(q-1)$

Possible values of $(p-1)$ and $(q-1)$ are:

$4,6,10,12,16$

The possible values for $K+1$ (formed by multipling two distinct values for $(p-1)$ and $(q-1)$) are:

$24,40,48,60,64,72,96,120,160,192$

So the possible values of $K$ are:

$23,39,47,59,63,71,95,119,159,191$

The only answer choice on this list is $119 \Rightarrow C$

Note: once we apply the factoring trick we see that, since $p-1$ and $q-1$ are even, $K+1$ should be a multiple of $4$.

These means that only $119 \Rightarrow C$ and $231 \Rightarrow E$ are possible.

We can't have $(p-1) \cdot (q-1)=232=2^3\cdot 29$ with $p$ and $q$ below $18$. Indeed, $(p-1) \cdot (q-1)$ would have to be $2 \cdot 116$ or $4 \cdot 58$.

But $(p-1) \cdot (q-1)=120=2^3\cdot 3 \cdot 5$ could be $2 \cdot 60,4 \cdot 30,6 \cdot 20$ or $10 \cdot 12.$ Of these, three have $p$ and $q$ prime, but only the last has them both small enough.

See also

2000 AMC 12 (Problems • Answer Key • Resources)
Preceded by
Problem 5 		Followed by
Problem 7
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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