Question

A train accelerates uniformly from 36 kilometer per hour
  to 72 kilometer per hour in 20 second. find the distance travelled by it.

Answer 1

36km/h = 10 m/s
72 km/h = 20 m/s
v = u +at
20 = 10 + a x 20
a = 1/2 m/s²
s = ut + 1/2at²
s= 10 x 20 + 1/2 x 1/2 x 20 x 20
s = 200 + 100
s = 300 m
so distance travelled = 300 m or 0.3 km

Answer 2

Distance traveled = average velocity * time duration
     = 1/2 * (72 + 36 ) * 1000/3600  meters/sec  * 20
     =  300 meters