Time, Speed, Distance Problem : PS Archive
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# Time, Speed, Distance Problem

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Manager
Joined: 19 Aug 2009
Posts: 79
Followers: 1

Kudos [?]: 223 [0], given: 46

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09 Sep 2009, 12:20
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Question Stats:

100% (02:18) correct 0% (00:00) wrong based on 3 sessions

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Pls explain
Q
John fires 2 bullets from the same place at an interval of 12 minutes but Jerry sitting in a train approaching the place hears the sound report 11 mins 30 secs after the 1st.What is the approximate speed of train( if sound travels at the speed of 330 m/s) ?
1. 660/23 m/s
2. 220/7 m/s
3. 330/23 m/s
Senior Manager
Joined: 23 Jun 2009
Posts: 361
Location: Turkey
Schools: UPenn, UMich, HKS, UCB, Chicago
Followers: 5

Kudos [?]: 130 [1] , given: 80

Re: Time, Speed, Distance Problem [#permalink]

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09 Sep 2009, 23:51
1
KUDOS
in 12 minutes sound travels. 12.60.330=237,600 metres.
in 11,30 minutes sound travels 11,5.60.330=227,700 metres.
That is exactly the same problem as two cars approaching each other.
Let train travels in constant speed. When jerry heard first bullet, train begins to travel (in constant speed, nothing matters.) There are 237.600 metres between them. After 11,5 minutes jerry heard the second sound. In this time, sound travelled 227,700 metres. If train had not travelled, sound would travel 237.600 metres. So train travelled 9.900 metres. Train makes this distance in 11,5 minutes=690 seconds. So, speed of train is 9.900/690=330/23 m/s.
Re: Time, Speed, Distance Problem   [#permalink] 09 Sep 2009, 23:51
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