A car starts from rest & move at constant acceleration of 5 m/s2 for some time,then move at const velocity for some time & retards to rest at constant decelerationof 5m/s2. If total time of it's journey is 25sec and Average velocity is 20m/s, time for which the car moves at constant velocity is?

Let car moves with 5m/s2 in t1 time,with constant velocity in t2 time,with constant retardation t3 time :-from equation of motion v=5t1,this v is initial velocity for constant retardation so v=-5t3 this way t1=t3 and by applying Vavg=Stotal/total time ,total distance is 500m.We get t2=25th, by graphical method 500=1/2×(25+t2)×5×(25-t2).we get t2=15 sec. Hence answer is 15 sec