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Any number that you choose can be the next number. It is easy to find a rule based on a polynomial of order 5 such that the first five numbers are as listed in the question followed by the chosen next number. There are also non-polynomial solutions. Short of reading the mind of the person who posed the question, there is no way of determining which of the infinitely many solutions is the "correct" one.

In this case the simplest solution, based on a polynomial of order 4 is

t(n) = (-77*n^4 + 874*n^3 - 3400*n^2 + 5321*n - 2676)/6 for n = 1, 2, 3, ... and accordingly, the next number is 693.

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9y ago
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Q: What is the next number in the series of 7 21 8 72 9?
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