A person travels along a straight road for the first $\frac{t}{3}$ time with a speed $v_1$ and for next $\frac{2t}{3}$ time with a speed $v_2.$ Then the mean speed $v$ is given by

Figure shows the graph of $x-$coordinate of a particle moving along $x-$axis as a function of time. Average velocity during $t=0$ to $6 \mathrm{s}$ and instantaneous velocity at $t=3 \mathrm{s}$ respectively, will be

Position-time graph for a particle is shown in figure. Starting from $t=0,$ at what time $t,$ the average velocity is zero?