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If two trains ,travelling at a speed of 72mph and 48 mph respectively,

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If two trains ,travelling at a speed of 72mph and 48 mph respectively, [#permalink]

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11 Mar 2017, 18:02
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If two trains ,travelling at a speed of 72mph and 48 mph respectively, are approaching each other from opposite direction; what is the distance between them 1 minute before the collision ?

A)2 miles
B)3 miles
C)4 miles
D)5 miles
E)none of the above.
[Reveal] Spoiler: OA

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Re: If two trains ,travelling at a speed of 72mph and 48 mph respectively, [#permalink]

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11 Mar 2017, 18:08
Relative speed between them = 48+72 MPH.

The distance they both will cover in a minute = (48+72)/60 = 2.

Hence A.

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Re: If two trains ,travelling at a speed of 72mph and 48 mph respectively, [#permalink]

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13 Mar 2017, 10:21
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Hi ziyuen,

When two entities (people, trains, etc.) are approaching one another, you have to consider the COMBINED speed that they're traveling - and the TOTAL DISTANCE that they're covering together. Here, we have two trains - traveling 72 mph and 48 mph - so combined, they travel 120 miles per hour. That means they travel a total of 120/60 = 2 miles per minute. Thus, when they're 1 minute from 'meeting', the distance between them is 2 miles.

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[Reveal] Spoiler:
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Re: If two trains ,travelling at a speed of 72mph and 48 mph respectively, [#permalink]

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14 Nov 2017, 07:43
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hazelnut wrote:
If two trains ,travelling at a speed of 72mph and 48 mph respectively, are approaching each other from opposite direction; what is the distance between them 1 minute before the collision ?

A)2 miles
B)3 miles
C)4 miles
D)5 miles
E)none of the above.

I call this a shrinking gap question.

When we ADD the speeds of the trains, we get 72 mph + 48 mph = 120 mph
This means, the gap SHRINKS at a rate of 120 miles per HOUR
We can also say that the gap SHRINKS at a rate of 2 miles per MINUTE

What is the distance between them 1 minute before the collision ?
Since the gap SHRINKS at a rate of 2 miles per MINUTE, we can conclude that, 1 minute before the collision, the trains will be 2 miles apart.

Answer: A

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Re: If two trains ,travelling at a speed of 72mph and 48 mph respectively, [#permalink]

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14 Nov 2017, 10:12
hazelnut wrote:
If two trains ,travelling at a speed of 72mph and 48 mph respectively, are approaching each other from opposite direction; what is the distance between them 1 minute before the collision ?

A)2 miles
B)3 miles
C)4 miles
D)5 miles
E)none of the above.

Distance is $$\frac{72+48}{60}$$ Miles/min = 2 Miles/Min

So, The distance between them 1 minute before the collision is 2 Miles, answer will be (A)
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Re: If two trains ,travelling at a speed of 72mph and 48 mph respectively,   [#permalink] 14 Nov 2017, 10:12
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If two trains ,travelling at a speed of 72mph and 48 mph respectively,

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