Answer:
20
Step-by-step explanation:
0 to 1 = a difference of 1
1 to 2 = a difference of 2-1 = 1
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2 to 4 = a difference of 2
4 to 6 = a difference of 2
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6 to 9 = a difference of 3
9 to 12 = a difference of 3
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12 to 16 = a difference of 4
16 to 20 = a difference of 4
