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71
A man sitting in a train is counting the pillars of electricity. The distance between two pillars is 60 meters, and the speed of the train is 42 km/hr. In 5 hours, how many pillars will he count?
Discuss
Answer & Solution
Answer: Option A
Solution:
Distance covered by the train in 5 hours
= (42 × 5) km
= 210 km
= 210000 m
∴ Number of pillars counted by the man
= $$\left( {\frac{{210000}}{{60}} + 1} \right)$$
= 3500 + 1
= 3501
72
A 120 meter long train is running at a speed of 90 km/hr. It will cross a railway platform 230 m long in :
Discuss
Answer & Solution
Answer: Option D
Solution:
Speed = $$\left( {90 \times \frac{5}{{18}}} \right)$$   m/sec = 25 m/sec
Total distance covered
= (120 + 230) m
= 350 m
∴ Required time
= $$\frac{{350}}{{25}}$$ seconds
= 14 seconds
73
A 50 meter long train passes over a bridge at the speed of 30 km per hour. If it takes 36 seconds to cross the bridge, what is the length of the bridge?
Discuss
Answer & Solution
Answer: Option B
Solution:
Speed = $$\left( {30 \times \frac{5}{{18}}} \right)$$  m/sec = $$\frac{{25}}{3}$$ m/sec
Time = 36 second
Let the length of the bridge be x meters.
Then, $$\frac{{50 + {\text{x}}}}{{36}}$$   = $$\frac{{25}}{3}$$
⇒ 3(50 + x) = 900
⇒ 50 + x = 300
⇒ x = 250 meters
74
A train takes 5 minutes to cross a telegraphic post. Then the time taken by another train whose length is just double of the first train and moving with same speed to cross a platform of its own length is :
Discuss
Answer & Solution
Answer: Option C
Solution:
Let the length of the train be x metres.
Time taken to cover x meters = 5 min
= (5 × 60) sec
= 300 sec
Speed of the train = $$\frac{{\text{x}}}{{300}}$$ m/sec
Length of the second train = 2x meters
Length of the platform = 2x meters
∴ Required time
$$\eqalign{ & = \left[ {\frac{{2{\text{x}} + 2{\text{x}}}}{{\left( {\frac{{\text{x}}}{{300}}} \right)}}} \right]{\text{sec}} \cr & = \left( {\frac{{4{\text{x}} \times 300}}{{\text{x}}}} \right){\text{sec}} \cr & = 1200\,{\text{sec}} \cr & = \frac{{1200}}{{60}}\,{\text{min}} \cr & = 20\,{\text{minutes}} \cr} $$
75
A train passes a station platform in 36 seconds and a man standing on the platform in 20 seconds. If the speed of the train is 54 km/hr, what is the length of the platform?
Discuss
Answer & Solution
Answer: Option B
Solution:
Speed = $$\left( {54 \times \frac{5}{{18}}} \right)$$  m/sec = 15 m/sec
Length of the train = (15 × 20) m = 300 m
Let the length of the platform be x meters
Then, $$\frac{{{\text{x}} + 300}}{{36}}$$  = 15
⇒ x + 300 = 540
⇒ x = 240 meters
76
A train speeds past a pole in 20 seconds and speeds past a platform 100 meters in length in 30 seconds. What is the length of the train?
Discuss
Answer & Solution
Answer: Option D
Solution:
Let the length of the train be x meters and its speed be y m/sec.
Then, $$\frac{{\text{x}}}{{\text{y}}}$$ = 20
⇒ y = $$\frac{{\text{x}}}{{20}}$$
∴ $$\frac{{{\text{x}} + 100}}{{30}}$$  = $$\frac{{\text{x}}}{{20}}$$
⇒ 30x = 20x + 2000
⇒ 10x = 2000
⇒ x = 200 meters
77
The time taken by a train 180 m long, travelling at 42 kmph, in passing a person walking in the same direction at 6 kmph, will be
Discuss
Answer & Solution
Answer: Option A
Solution:
Speed of train relative to man
= (42 - 6) kmph = 36 kmph
= $$\left( {36 \times \frac{5}{{18}}} \right)$$  m/sec
= 10 m/sec
∴ Time taken to pass the man
= $$\frac{{180}}{{10}}$$ sec
= 18 sec
78
Two trains 200 meters and 150 meters long are running on parallel rails in the same direction at speed of 40 km/hr and 45 km/hr respectively. Time taken by the faster train to cross the slowed train will be:
Discuss
Answer & Solution
Answer: Option D
Solution:
Relative speed = (45 - 40) km/hr = 5 km/hr
= $$\left( {5 \times \frac{5}{{18}}} \right)$$  m/sec
= $$\frac{{25}}{{18}}$$ m/sec
Total distance covered = Sum of lengths of trains = (200 + 150) m = 350 m
∴ Time taken
= $$\left( {350 \times \frac{{18}}{{25}}} \right)$$   sec
= 252 seconds
79
A train with 90 km/hr crosses a bridge in 36 seconds. Another train 100 meters shorter crosses the same bridge at 45 km/hr. What is the time taken by the second train to cross the bridge?
Discuss
Answer & Solution
Answer: Option D
Solution:
Let the lengths of the train and the bridge be x meters and y meters respectively.
Speed of the first train
= 90 km/hr
= $$\left( {90 \times \frac{5}{{18}}} \right)$$  m/sec
= 25 m/sec
Speed of the second train = 45 km/hr
= $$\left( {45 \times \frac{5}{{18}}} \right)$$  m/sec
= $$\frac{{25}}{2}$$ m/sec
Then, $$\frac{{{\text{x}} + {\text{y}}}}{{36}}$$ = 25
⇒ x + y = 900
∴ Required time
$$\eqalign{ & = \left[ {\frac{{\left( {{\text{x}} - 100} \right) + {\text{y}}}}{{\frac{{25}}{2}}}} \right]{\text{sec}} \cr & = \left[ {\frac{{\left( {{\text{x}} + {\text{y}}} \right) - 100}}{{\frac{{25}}{2}}}} \right]{\text{sec}} \cr & = \left( {800 \times \frac{2}{{25}}} \right){\text{sec}} \cr & = 64\,{\text{sec}} \cr} $$
80
A train 125 m long passes a man, running at 5 kmph in the same direction in which the train is going, in 10 seconds. The speed of the train is:
Discuss
Answer & Solution
Answer: Option B
Solution:
Speed of the train relative to man
$$\eqalign{ & = \frac{{125}}{{10}}{\text{m/sec}} \cr & = \frac{{25}}{2}{\text{m/sec}} \cr & = \left( {\frac{{25}}{2} \times \frac{{18}}{5}} \right){\text{m/sec}} \cr & = 45\,{\text{km/hr}} \cr} $$
Let the speed of the train be x kmph.
Then, relative speed = (x - 5) kmph
∴ x - 5 = 45 or
x = 50 km/hr