Question 845949
<pre>
3, -6 , 12, 4, 20 

Start with 3

 3 -  9 = -6
-6 + 18 = 12
12 -  8 =  4
 4 + 16 = 20
<font color="red">20 -  7 = 13
13 + 14 = 27
27 -  6 = 21</font>

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7, 16, 8, 27, 9 

7
2×8 = 16
8
3×9 = 27
9
<font color="red">4×10 = 40
10
5×11 = 55</font>

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7, 26, 101, 400

Start with 7

  7×4-2 =   28-2 =  26
 26×4-3 =  104-3 = 101
101×4-4 =  404-4 = 400
<font color="red">400×4-5 = 1600-5 = 1595</font> 

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7, 21, 8, 72, 9

Start with 7
 7
 7×3<sup>1</sup> =   7×3 =  21
 8 
 8×3<sup>2</sup> =   8×9 =  72
 9
<font color="red"> 9×3<sup>3</sup> =  9×27 = 243
10
10×3<sup>4</sup> = 10×81 = 810</font>

Edwin</pre>