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Given the series below, can you find its formula and hence the missing number?

3, -1, 7, 8, 20, 35, 43, 76, ??, 131, 159

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$76$.

because:

$a_{n+1}=n\cdot p_n-a_n$, with $a_0=3$ and $p_n$ the $n^{th}$ prime number.

So:
$3-1=2=1\cdot2$
$-1+7=6=2\cdot3$
$7+8=15=3\cdot5$
$8+20=28=4\cdot 7$
etc..
Meaning that the missing term is $8\cdot19=152-76=76$.
And to check, $76+131=207=9\cdot23$.

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