Acceleration = Slope of v - t graph

$\therefore \ For\ 0\lt t\ \lt 5\ sec, \ \ \ a=\dfrac{5-5}{5-0}=0\\ \therefore \ For \ 5\lt t\ \lt 15\ sec, \ \ \ a=\dfrac{20-5}{15-5}=1.5\ m/s^2$

$S_0=0\\ \therefore S_5=S_0+(5\times 5)=25\\ \therefore S_{15}=S_5+\Bigg[(10\times 5)+\dfrac{1}{2}(10\times 15)\Bigg]=150\ m$