Aptitude - Problems on Trains - Discussion

Why we take 8 sec why don't we consider 20 sec?

1) Train cross the pole in 8 sec.

Speed = Distance/Time.

Assume the train speed is x and length of train be y.

x = y/8 or y = 8x.

2) Train cross the bride which is 264mts long 20 sec.

x = (264+y)/20.

20x = 264+8x because y = 8x in above.

Solving the above equation we get speed is 22m/sec.

Finally convert the m/sec to km/hr.

22*18/5 = 79.2 km/hr.

Don't go complex one, we need Speed = Distance/Time.

Total distance = 284.

Time taken to cross is 12 sec (20-8).

Speed = 22 m/s or 79.2 km/hr.

Last one is very smart and short method. Actually I was also doing this. But I forgot to convert m/s to km/hr. For that my answer was 22 m/s. So I thought that I was wrong. Because the answer given in km/hr.

Total distance = 284.

And time taken (20-8) = 12 because it is in same direction.

And now Speed = Distance/Time.

= 284/12.

= 22 m/s.

= 22*18/5 km/hr.

= 79.2 km/hr.

Relative time = 20 - 8
= 12
Then,

Speed = Distance/ time
= 264/12
= 22 m/sec
= 22 *15/5 km/h.
= 79.2 km/ h.

Answer = 79.2 km/ h.

Hi, I can't understand. Why are we taking time as the difference of both?

I am not getting the answer when we proceed it with the same step as the previous problem.

Simple answer!

A train moves past a telegraph post and a bridge 264 m long in 8 seconds and 20 seconds respectively. What is the speed of the train?

Train moves past a Tpost in 8 sec, 264 m long bridge in 20 sec (difference is 12 sec) now calculate the length of the train = 8 x 264 / 12 = 178. mtrs.

Formula -> Time= distance/speed
=> 8 = 178/speed (178/8 = 22 meters/seconds).
Now convert 22 meter/sec to Km/hr by multiplying 22 x 18/5 or (3600 sec/1000 meters) = 79.2.

@Hussaian.

To convert m/s to km/be, we have the formula a* 18/5 because to convert the metre per sec to km/h we multiply the number to b converted by 3600/1000 by simplifying this we get 18/5.

We directly do speed=264/12.
=22*18/5. And we get the answer.