Chapter 11 - Review - Page 668: 47

$S_6=150$

The given sequence $ 15,19,23,... $ is an arithmetic sequence with $a_1=15$ and $d=4$. Using $S_n=\dfrac{n}{2}(2a_1+(n-1)d)$, then, \begin{array}{l} S_6=\dfrac{6}{2}(2\cdot15+(6-1)4)\\ S_6=3(30+20)\\ S_6=150 .\end{array}