Aptitude - Problems on Trains - Discussion

=> x+800/60 = 78 * 5/18,
=> x+800/60 = 65/3,
=> x+800 = 65 * 60/3,
=> x+800 = 1300,
=> x = 1300-800,
=> x = 500.

Why we are adding the length of train and length of tunnel? Anyone please explain to me.

@Agggy.

we add the length of the the train and tunnel because the train is said to have crossed the tunnel if and only if its entire part including the the very last end of it leaves the tunnel.

so if the train has crossed the tunnel in 1min then it would have covered the entire length of the tunnel and a certain distance outside the tunnel (the distance covered by the train until the very last end of the train leaves the tunnel).

So,
that 'certain distance' = length of the train.
.A______x______.B.______800______.C.

where x is the length of the tunnel and the length of the train is 800 m.
the total distance covered by the train is x + 800.

DISTANCE = X+800 m
SPEED = 78 km/hr
TIME = 60sec.

convert speed from km/hr to m/s.
SPEED = 78x1000/60 x 60.
SPEED = 65m/s.

distance /speed = time.
X + 800/65 = 60,
X + 800 = 60x65/3,
X + 800 = 3900/3,
X + 800 = 1300,
X = 1300 - 800,
X = 500.

I do not understand what is the meaning of crossing a tunnel you understand. I think it should mean the moment train head enters the tunnel till its TAIL comes out one can not consider the train is out of tunnel if its head just come out.

Thanks all for explaining the answer.

How to solve this? please explain to me.

How the 5/18 came?

If there will be 75 km/h then what will be the answer?

How the number 5/18 came? please clarify me.

What if we turn the speed into m/s first, like.

78 x (5/18)=> 65/3 => 15,
then use formula D = S*T.
then we get,
D+800 = 15 * 60.
D+800 = 900,
D = 100,

Is this wrong ?
why 2 different answer?