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Find relation between the following numbers: enter image description here

we can use only Elementary arithmetic (addition, subtraction, multiplication, and division) . I find this puzzle in my old book .I think we have Arithmetic progression , because in the first shape (up left) we have 5+6+1=12 ,1+6+9=16 ,9+6+5=20 so we have sequence:12,16,20 so common difference of successive members is $d=4$ for second shape we have:13,16,19 so $d=3$ for third shape:18,20,22 ,$d=2$ finally for last shape we have $d=1$ but this is impossible. I think my answer is wrong because we delete center number in this puzzle and we must use center numbers in all shapes.

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I'm not certain what the rules are for this, but using subtraction only you can have a progression where the result is figure 1 leaves you with 1, figure 2 with 2, figure 3 with 3, and figure 4 with 4:

1: 9-6 = 3; 5-3 = 2; 2-1 = 1
2: 8-4 = 4; 7-4 = 3; 3-1 = 2
3: 7-6 = 1; 9-1 = 8; 8-5 = 3

and lastly...

4: 5-1 = 4; 4-1 = 3; 7-3 = 4

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