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I really have no clue how to solve this. I see no pattern at all. I treid difference, multiplication yet I see no pattern.

Fill in the dots:

11 21 23 17 ...

Possible answers: 11 3 9 1

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    $\begingroup$ If you don't mind my asking where is this from? Are there more like it? $\endgroup$ – hat Jul 25 '18 at 13:27
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I guess the answer is

3

Reason:

The differences of successive pairs of numbers are 10, 2, -6.
This decreases by 8 at each step. The next difference would then be -14, making the next number 17-14=3.

This method essentially fits a quadratic through the points represented by the numbers. The fact that it goes through all four given points when three are enough to specify it gives some credence that this was intended.

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