Question

400 m long train crosses a flag post in half a minute. What is its speed?

Answer 1

Distance covered by the train = Length of the train = 400 m. 

Time taken = Half Minute = 30 sec.

Then,

Speed = Distance/ Time =  400/30  =  40/3 = 13.33 m/sec.

Answer 2

Hey there!
Width of the flag post is neglected here.

To cross the flag pole, Total length of train must go pass flag post

Distance travelled by train = 400m .

Time taken = 1/2 min = 30 seconds.

Speed of the train = 400/30 = 40/3 = 13.33m/s

Hope you are helped!