Answer:
Explanation:
trial and improvement
4, 5, 6
4 x 3= 12
6 x 2= 12 not right
6, 7, 8
6 x 3=18
8 x 2= 16 not right
9, 10, 11
9 x 3= 27
11 x 2= 22 not right
11, 12, 13
11 x 3= 33
13 x 2= 26 not right
13, 14, 15
13 x 3= 39
15 x 2= 30
39-30=9
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