The velocity and time graph for a particle moving in a straight line s shown in the figure. Then, the average velocity between t = 4 s and t=6 s is____

Average velocity = total displacement/total time

V_avg for time interval 4 to 6 secs is 

V_avg = (area under the curve from t = 4 to 6)/2secs

Curve can be written as v = (15/6)t

Area = ${\int }_4^{6}15/6tdt$ = $\left|15t^2/12\right|$ = 25

V_avg = 25/2 = 12.5m/s