What should the next number be in the series 100 6 104 88 120 56?
Any number that you choose can be the next number. It is easy to find a rule based on a polynomial of order 6 such that the first six 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.The simplest rule, based on a polynomial of order 5 isU(n) = (-129*n^5 + 2325*n^4 - 15885*n^3 + 50715*n^2 - 73706*n + 38680)/20 for n = 1, 2, 3, ...and, accordingly, the next number is 1328.