Problems on Trains Questions and Answers Part-7

1. Two trains of lenths 120 m and 90 m are running with speed of 80 km/hr and 55 km/hr respectively towards each other on parallel lines. If they are 90 m apart, after how many seconds they will cross each other?
a) 5.6 sec
b) 7.2 sec
c) 8 sec
d) 9 sec.

Answer: c
Explanation:
\eqalign{ & {\text{Relative speed}} \cr & {\text{ = (80 + 55)km/hr}} \cr & {\text{ = 135 km/hr}} \cr & {\text{ = }}\left( {135 \times \frac{5}{{18}}} \right)m/\sec \cr & = \left( {\frac{{75}}{2}} \right)m/\sec \cr & {\text{Distance covered}} = (120 + 90 + 90)m \cr & {\text{ = 300m}} \cr & {\text{Required time}} \cr & {\text{ = }}\left( {300 \times \frac{2}{{75}}} \right)\sec \cr & = 8\sec \cr}

2. Two trains are coming from opposite directions with speed of 75 km/hr and 100 km/hr on to parallel tracks. At some moment the distance between them is 100km. After T hours, distance between them is again 100 km. T is equal to?
a) 1 hr
b) $$1\frac{1}{7}$$ hr
c) $$1\frac{1}{2}$$ hr
d) 2 hr

Answer: b
Explanation:
\eqalign{ & {\text{Relative speed}} = (75 + 100)km/hr \cr & {\text{ = 175 km/hr}} \cr & {\text{Time taken to cover 175 km}} \cr & {\text{at relative speed = 1 hr}} \cr & {\text{T = Time taken to cover 200 km}} \cr & {\text{ = }}\left( {\frac{1}{{175}} \times 200} \right)\, \text{hr} \cr & = \frac{8}{7}\, \text{hr} \cr & = 1\frac{1}{7}\, \text{hr} \cr}

3. A train, 240 m long, crosses a man walking alone the line in opposite direction at the rate of 3 kmph in 10 seconds. The speed of the train is?
a) 63 kmph
b) 75 kmph
c) 83.4 kmph
d) 86.4 kmph

Answer: c
Explanation:
\eqalign{ & {\text{Speed of the train relative to man}} \cr & {\text{ = }}\left( {\frac{{240}}{{10}}} \right){\text{m/sec}} \cr & {\text{ = 24 m/sec}} \cr & {\text{ = }}\left( {24 \times \frac{{18}}{5}} \right){\text{ km/sec}} \cr & {\text{ = }}\frac{{432}}{5}{\text{km/hr}} \cr & {\text{Let the speed of the train be x kmph}}{\text{.}} \cr & {\text{Then relative speed = }}\left( {x + 3} \right){\text{kmph}} \cr & x{\text{ + 3 = }}\frac{{432}}{5} \cr & x = \frac{{432}}{5} - 3 \cr & x = \frac{{417}}{5} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\, = 83.4\,{\text{kmph}} \cr}

4. Two trains of equal length are running on parallel lines in the same directions at 46 km/hr and 36 km/hr. The faster train passes the slower train in 36 seconds. The length of each train is?
a) 50 m
b) 72 m
c) 80 m
d) 82 m

Answer: a
Explanation:
\eqalign{ & {\text{Let the length of each train be }}x{\text{ metres}} \cr & {\text{Then distance covered}} \cr & {\text{ = 2x metres}} \cr & {\text{Relative speed}} \cr & {\text{ = (46}} - {\text{36)km/hr}} \cr & {\text{ = }}\left( {10 \times \frac{5}{{18}}} \right)m/\sec \cr & = \left( {\frac{{25}}{9}} \right)m/\sec \cr & \frac{{2x}}{{36}} = \frac{{25}}{9} \Leftrightarrow 2x = 100 \Leftrightarrow x = 50 \cr}

5. Two trains of equal lengths takes 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 miters, in what time ( in seconds) will they cross each other traveling in opposite direction?
a) 10
b) 12
c) 15
d) 20

Answer: b
Explanation:
\eqalign{ & {\text{Speed of the train}} \cr & {\text{ = }}\left( {\frac{{120}}{{10}}} \right){\text{ m/sec}} \cr & {\text{ = 12 m/sec}} \cr & {\text{Speed of the second train}} \cr & {\text{ = }}\left( {\frac{{120}}{{15}}} \right){\text{ m/sec}} \cr & {\text{ = 8 m/sec}} \cr & {\text{Relative speed}} \cr & {\text{ = (12 + 8)m/sec}} \cr & {\text{ = 20 m/sec}} \cr & {\text{Required time}} \cr & {\text{ = }}\frac{{\left( {120 + 120} \right)}}{{20}}\,\sec \cr & = 12\,\sec \cr}

6. 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?
a) 100 meters
b) 150 meters
c) 180 meters
d) 200 meters

Answer: d
Explanation: 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

7. 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
a) 18 sec
b) 21 sec
c) 24 sec
d) 25 sec

Answer: a
Explanation: 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

8. 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:
a) 72 seconds
b) 132 seconds
c) 192 seconds
d) 252 seconds

Answer: d
Explanation: 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

9. 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?
a) 61 seconds
b) 62 seconds
c) 63 seconds
d) 64 seconds

Answer: d
Explanation: 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}.

10. 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:
a) 45 km/hr
b) 50 km/hr
c) 54 km/hr
d) 55 km/hr

Answer: b
Explanation: 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