Question 1017610
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One possible rule is:

If we add the 2nd and 3rd numbers and then 
subtract 6, we always get twice the first number.

Guess the rule.
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2-4-6 satisfies the rule
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because if we add the 2nd and 3rd numbers, 
4+6, we get 10, 
then if we subtract 6, we get 4,
and 4 is twice the 1st number 2
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2-4-5 does not
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because if we add the 2nd and 3rd numbers, 
4+5 we get 9, 
then we subtract 6 and get 3,
and 3 is NOT twice the first number 2
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2-5-5 does
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because if we add the 2nd and 3rd numbers,
5+5 we get 10, 
then we subtract 6 and get 4,
and 4 is twice the first number 2
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3-4-5 does not
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because if we add the 2nd and 3rd numbers, 
4+5 we get 9, 
then we subtract 6 and get 3,
and 3 is NOT twice the first number 3
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3-6-6 does
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because if we add the 2nd and 3rd numbers, 
6+6 we get 12, 
then we subtract 6 and get 6,
and 6 is twice the 1st number 3
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4-5-6 does not
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because if we add the 2nd and 3rd numbers, 
5+6 we get 11, 
then we subtract 6 and get 5,
and 5 is NOT twice the first number 4.
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4-5-7 does not
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because if we add the 2nd and 3rd numbers, 
5+7 we get 12, 
then we subtract 6 and get 6,
and 6 is NOT twice the first number 4.
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4-5-9 does 
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because if we add the 2nd and 3rd numbers,
 5+9 we get 14, 
then we subtract 6 and get 8,
and 8 is twice the first number 4.

The rule works.  But there are probably many other
rules that would do.

Edwin</pre>