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The next number is 18.
This is obtained by fitting the quartic polynomial
t(n) = (-n4 + 12n3 - 47n2 + 84n - 36)/4 for n = 1, 2, 3, etc.
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Earn +20 ptsQ: What is the next number in the series 3 6 9 15 21?
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Which number should come next in the series 1-3-6-10-15-?
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Which number completes the series 1 3 6 10 15?
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