Question

A train 400m long is running at a speed 72 km/ HR . How much time does it to cross a telegraph post ??

Answer 1

Answer:

$\text{ \large \: 20 \: seconds}$

Step-by-step explanation:

$\sf \: The \: length \: of \: train \: is \: given \: in \: meters \: .$

$\sf \: Hence, \: we \: shall \: convert \: its \: speed \: in \: \frac{m}{s}$

$\sf \green{speed} \: = 72 \frac{km.}{hr.} = \frac{72 \times 1000}{3600}$

$\green{ \implies \boxed{\: 20 \frac{m.}{sec.}}} \:$

To cross Telegraph Post, The train will cover a distance equal to its length , that is 400m.

Let the required time = x sec.

Thus we have,

$\boxed{ \text{\: distance \: (in \: m )} = \boxed{20 \: \: \: \: \: 400}}$

$\boxed{ \text{\: time \: (in \: sec. )} = \boxed{ \: \: 1 \: \: \: \: \: \: \: \: \: \: x}}$

$\sf \: Clearly , \: It \: is \: case \: of \: \bold{direct \: Variation}$

$\sf \frac{20}{1} = \frac{400}{x}$

$\sf \: x \: = \frac{400}{20} = \boxed {\large{ \green{20}} }$

Hence, time taken by the train to cross Telegraph Post is 20 seconds.

Answer 2

Hi !

Speed of the train (S) = 72 km/hr

=20m/sec

Distance =. 400m

Time =. ?

therefore

$speed = \frac{distace}{time} \\ 20 = \frac{400}{time} \\ time = \frac{400}{20} \\ time = 20sec$

Hence,

If a train of 400 m long is running at a speed of 72 km/hr, then it cross a telegraph post at a time of 20 sec.