Aptitude - Problems on Trains - Discussion

Hi,

Resolving the fractions here is 7/2 + 1/4 is equal to 15/4, how

Since (x/y+a/b) is (x*b/y*b) + (y*a/y*b) = (x*b + y*a)/y*b

Therefore

(7/2 + 1/4 )=> (7*4/2*4) + (2*1/2*4)=> (28/8 + 2/8)=> (30/8)=> 15/4.

How can do it (15/4*75) hrs,
Because 15/4 unit-mils.
and
75 unit-mph.

Then how can possible (15/4*75)hrs?

Hi @Arun kumar. This method is so confusing can you please tell me other method to convert into 15/4.

Here the speed given in metre/hr and distance given in mile then how it possible to evaluate time without converting length in same unit.

Ya as train enters time starts so distance 1/4 and as train leave from tunnel distance covered is more 1/4 since it has covered xtra 1/4 distance after coming out of tunnel.

---------front[ (tunnel) ]rear------------
1/4 3/2 1/4

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.