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Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds

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Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds  [#permalink]

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New post 08 Jan 2020, 12:20
1
00:00
A
B
C
D
E

Difficulty:

  55% (hard)

Question Stats:

46% (01:29) correct 54% (01:21) wrong based on 24 sessions

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Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds respectively. Length of train M is half of length of train N. What is the respective ratio between speed of train M and train N?
(a) 2 : 3
(b) 1 : 2
(c) 2 : 1
(d) 3 : 2
(e) None of these

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Re: Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds  [#permalink]

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New post 08 Jan 2020, 12:53
1
I would say none of these, but Im not sure.
The reason for E being, the ratio of time is reciprocal of the speed and vice versa.

In this case, M cross the pole in 25sec and B in 75sec.

Ration of M:N=1:3,so reciprocal of time is speed=3:1, which is not in the options.

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Re: Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds  [#permalink]

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New post 08 Jan 2020, 13:02
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Let the lengths of the trains M and N be x and 2x, respectively.

Both the trains cover the distance equal to their respective lengths when they cross the pole

Speed of train M = Distance/Time = \(\frac{x}{25}\)
Speed of train N = Distance/Time =\(\frac{ 2x}{75}\)
Therefore, Required ratio of speeds = \( \frac{x}{25}\) * \(\frac{75}{x}\) = \(\frac{3 }{ 2}\)

Hence the ratio is 3:2

Answer must be D
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Re: Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds  [#permalink]

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New post 08 Jan 2020, 20:36
TheNightKing wrote:
Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds respectively. Length of train M is half of length of train N. What is the respective ratio between speed of train M and train N?
(a) 2 : 3
(b) 1 : 2
(c) 2 : 1
(d) 3 : 2
(e) None of these


Explantion:

Let the lengths of the trains A and B be a and 2a, respectively.

When a train crosses a pole. it covers the distance equal to its length.

Therefore, Required ratio of speeds = a/25:2a/75 = 3:2

IMO-D
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Re: Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds  [#permalink]

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New post 20 Jan 2020, 20:37
Hi,

I am not getting one thing. If we take the length of M as d/2 and length of N as d the answer is not coming properly. Cn anyone explain it to me.
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Re: Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds   [#permalink] 20 Jan 2020, 20:37
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Two trains M and N crosses a pole in 25 seconds and 1 min 15 seconds

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