Aptitude - Problems on Trains - Discussion

Time taken=15/(4*75) how its comes.

Because t=d*s means (15 * 75)/4 only possible am confused.

How come to 15 could you explain?

It is 3 min. For sure as the train enters into the tunnel distance inside the tunnel counts and not the back part of train.

So when rear emerges, only 1/4 is added to 7/2 and not 1/4+1/4. So 3 min. Is absolutely correct.

How come 15/4 could you explain?

Ya friends let me explain it in detail, hope this will help you,

Given the speed of train = 75 mph which means (miles per hr). Because logically speaking there will be no train moving only 75 meters per one hour.

Tunnel length = 3 1/2.
= (3*2+1)/2 = 7/2 miles.
Train length = 1/4 mile.

So now as every thing is given in miles there is no need of any conversion. Now let us go to the problem.

For a train of length 'l' to cross a stationary object of length 'b' the time taken is equal to the time taken to cover (l+b) miles;

= 7/2+1/4.
Taking lcm as 4.
= ( (7*2) +1) /4.
=15/4.

Now the main confusion is to add another 1/4 distance or not. Here is the solution. It is not needed because.

At the start of train we are considering at entrance of the tunnel, so train length is not to be considered at starting end. But at the rear end of the tunnel it was said that trains rare end is to be passed, so rain length is to be added only once.

Now Speed = Distance/Time,
So, Time = Distance/Speed,

Now from above calculation.
Distance = 15/4 miles,
Speed = 75 miles per hour = 15/4 miles.

Time=-----------------75 miles/hr.

T = (15/ (75*4) ) hr.
T = 1/20 hrs.
T = 1/20*60 mins.
T = 3 minutes.

So the answer is 3 minutes.

Why we not convert 75 kmph to m/sec?

Speed is given in meter/hr.

How come its taken with distance in miles and calculated (15/4)/75? How that possible different units?

Can any body explain, in the particular question my doubt is length is miles per hour and also unit of speed is also given in miles. Whether the length-speed or length-time is to be same unit of measurement to calculate the third?

I was thinking length-time-speed should be either Kilometer-hour-Kph or meter-seconds-meter per second.

How did you know that the time found was in terms of hours?

Question itself says, time from front of train enters to rear emerges.

So distance traveled should be 7/2 + 1/4 + 1/4 = 4.