$\mathrm{Avg}.\mathrm{Av}.=\frac{\mathrm{change}\mathrm{in}\mathrm{velocity}}{\mathrm{total}\mathrm{time}\mathrm{taken}}$

$=\frac{{\mathrm{v}}_2-{\mathrm{v}}_1}{\mathrm{t}}=\frac{{\mathrm{tan\theta }}_2-{\mathrm{tan\theta }}_1}{\mathrm{t}}=0$

At position A, slope, which is velocity of particle is 90° tan 90° = infinite. at T = 10 sec. slope is 0° = tan 0 = 0. So, velocity is decreasing from A to t = 10 sec. Thus retarding from 0 to 10 sec.
Vice-versa is happening from t = 10 to t = 20 sec.

$\mathrm{Avg}.\mathrm{velocity}=\frac{{\mathrm{x}}_{\mathrm{f}}-{\mathrm{x}}_{\mathrm{i}}}{\mathrm{total}\mathrm{time}\mathrm{taken}}=\frac{-100-100}{20}=-10\mathrm{m}/\mathrm{s}$