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21 Apr 2020, 07:40
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Difficulty:
55%
(hard)
Question Stats:
65% (02:13) correct
35%
(02:15)
wrong
based on 257
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100-meter long train A and 150-meter long train B are moving towards each other, at 54 km/h and 36 km/h respectively. If a person in standing still in train A, how long will it take him to pass train B if the distance between him and the train B is 150 meters?
A. 15 seconds
B. 12 seconds
C. 10 seconds
D. 6 seconds
E. 5 seconds
A. 15 seconds
B. 12 seconds
C. 10 seconds
D. 6 seconds
E. 5 seconds
21 Apr 2020, 11:07
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Relative velocity Of person wrt train B = (54+36)= 90 kmh or 25m/s
Length of train B = 150 m; distance between him and train B = 150m
Time taken by him to cross train B = \(\frac{150+150}{25} = 12 sec\\
\)
Length of train B = 150 m; distance between him and train B = 150m
Time taken by him to cross train B = \(\frac{150+150}{25} = 12 sec\\
\)
Bunuel
21 Apr 2020, 11:38
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To be able to solve this problem, you require to know the concept of relative speed. For those who unaware:
What is relative speed? Basically, it is speed of an object relative to another object. For example- Person A is standing still and B is moving at a speed of 5 mph. What is their relative speed to each other? That is right, 5 mph! But if they started to move to opposite directions: A moves at 2 mph and B at 5 mph in opposite directions, they moving away from each other at 7 mph, therefore they relative speed is 7 mph. If they started to move toward each other: A at 2 mph, and B at 5 mph, they are still covering 7 mph. If they are moving in the same direction, say, A at 2 mph and B at 5 mph, they relative speed to each other is 3 mph. Hope it makes sense.
Back to our question. Basically, we need to find the relative speed of train A to B, which is 90, since together they cover 90 kmph. Now we need to convert 90 kmph to meters per second----->25 meters per second
(For those who struggle with conversion problems----->https://gmatclub.com/forum/conversion-problems-175012.html#p1375755 )
Since the lenght of train B is 150m, we need to find the time trains cover this distance------>150/25=6 seconds
Kudos, if it helped)
What is relative speed? Basically, it is speed of an object relative to another object. For example- Person A is standing still and B is moving at a speed of 5 mph. What is their relative speed to each other? That is right, 5 mph! But if they started to move to opposite directions: A moves at 2 mph and B at 5 mph in opposite directions, they moving away from each other at 7 mph, therefore they relative speed is 7 mph. If they started to move toward each other: A at 2 mph, and B at 5 mph, they are still covering 7 mph. If they are moving in the same direction, say, A at 2 mph and B at 5 mph, they relative speed to each other is 3 mph. Hope it makes sense.
Back to our question. Basically, we need to find the relative speed of train A to B, which is 90, since together they cover 90 kmph. Now we need to convert 90 kmph to meters per second----->25 meters per second
(For those who struggle with conversion problems----->https://gmatclub.com/forum/conversion-problems-175012.html#p1375755 )
Since the lenght of train B is 150m, we need to find the time trains cover this distance------>150/25=6 seconds
Kudos, if it helped)
