In the above graph, we can see that the acceleration of the particle is positive for the whole duration. Therefore, velocity will keep on increasing. The velocity will be maximum at $t=15 \mathrm{s}$.

Area under acceleration time graph gives velocity. Therefore,

$v_{15}=\frac{1}{2}\times 10\times 15=75 \mathrm{m} {\mathrm{s}}^{-1}$.