Question

the train a 300 meters long passes a standing man in 15 seconds. the speed of the train is :

Answer 1

distance covered by train front( lets engine ) is equal to length of the train.
so distance = 300 m
time taken = 15 second

so speed = 300/15
=20 m/s

Answer 2

Here distance will be the length of the train
$speed = \frac{distance}{time} \\ = \frac{300}{15} \\ = 20 \: m/s$