The answer is 10050.

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This is an arithmetic series with $\displaystyle a_1 = 5$ and $\displaystyle d = 8$ .

$\displaystyle S_n = \frac n2 (a_1 + a_n) = \frac n2 \Bigg[a_1 + \bigg(a_1 + d(n - 1)\bigg)\Bigg]$

$\displaystyle S_{50} = \frac {50}{2} \bigg(5 + 5 + 8(49)\bigg)$

$\displaystyle = 10050$