Aptitude - Problems on Trains - Discussion
Discussion Forum : Problems on Trains - General Questions (Q.No. 17)
17.
A train 800 metres long is running at a speed of 78 km/hr. If it crosses a tunnel in 1 minute, then the length of the tunnel (in meters) is:
Answer: Option
Explanation:
| Speed = |  |
78 x | 5 |  |
m/sec | = | |
65 | |
m/sec. |
| 18 | 3 |
Time = 1 minute = 60 seconds.
Let the length of the tunnel be x metres.
| Then, | |
800 + x | |
= | 65 |
| 60 | 3 |
3(800 + x) = 3900
x = 500.
Discussion:
52 comments Page 6 of 6.
Agggy said:
3 months ago
Why we are adding the length of train and length of tunnel? Anyone please explain to me.
Rishika V said:
2 months ago
@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.
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.
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