Answer
$1110$
Work Step by Step
I know that $\sum_{k=1}^{n} (k)=\frac{n(n+1)}{2}.$
Hence $\sum_{k=1}^{20} (5k+3)=\sum_{k=1}^{20} (5k)+\sum_{k=1}^{20} (3)=5\sum_{k=1}^{20} (k)+20(3)=5\frac{20(20+1)}{2}+60=1050+60=1110$
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