Interval 3 (Greatest), Interval 2 (Least) Positive (Intervals $1$ and $2$), Negative (Interval 3 ) The average speed of a particle shown in the $x-t$ graph is obtained from the slope of the graph in a particular interval of time. It is clear from the graph that the slope is maximum and minimum restively in intervals 3 and 2 respectively. Therefore, the average speed of the particle is the greatest in interval 3 and is the least in interval $2 .$ The sign of average velocity is positive in both intervals 1 and 2 as the slope is positive in these intervals. However, it is negative in interval 3 because the slope is negative in this interval.