The following figure shows x-t graph of a particle. Find the time t such that the average velocity of the particle during the period 0 to t is 0. 

At t = 0 sec, displacement S= 20 m

On drawing a line from point B down to x-axis we get that At t = 12 sec also S = 20 m.

Hence for time interval between 0 sec to 12 sec , change in displacement = 0

We know that average velocity = displacement / time

= 0

Hence in the time interval between 0 to t = 12 sec ,the average velocity of particle =0 .