Answer
165
Work Step by Step
$a_{n}=a+\left( n-1\right) d$
$a_{2}=a+\left( 2-1\right) d=a+d$
$a_{5}=a+\left( 5-1\right) d=a+4d$
$a_{5}-a_{2}=3d=9.5-8=1.5\Rightarrow d=0.5$
$a_{2}=a+d\Rightarrow 8=a+0.5\Rightarrow a=7.5$
$S_{n}=\dfrac {n}{2}\left( 2a+\left( n-1\right) d\right) =\dfrac {15}{2}\left( 2\times 7.5+\left( 15-1\right) \times 0.5\right) =165$
