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.
2-4-6 satisfies the rule
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
2-4-5 does not
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
2-5-5 does
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
3-4-5 does not
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
3-6-6 does
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
4-5-6 does not
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.
4-5-7 does not
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.
4-5-9 does
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
