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A searchlight on top of the watch-tower makes 3 revolutions

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Intern
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Joined: 09 Sep 2014
Posts: 6
A searchlight on top of the watch-tower makes 3 revolutions [#permalink]

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New post 08 Oct 2014, 16:29
A very literal solution...


One minute <-------------|---------------|---------------->
************************** 14-24 **********36-44*************

Segment at which darkness will intercede 5 seconds

Time between 15+9+15 = 13/20

& Closest answer is 3/4.....
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B
Joined: 02 Dec 2017
Posts: 2
Re: A searchlight on top of the watch-tower makes 3 revolutions [#permalink]

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New post 04 Feb 2018, 08:35
I approached this question as if the length of a minute is the length of a rope. Let's say we have a length of a rope-60cm. And every 20cm, there is a cut.
--------20--------20--------20
-----15-20-----15-20------15-20
Probability of man being in every 15 cm or less is calculated as follows. Overall there are 5+5+5 areas that the man should not stand, if it wants to be waiting at least 5 seconds (which are the areas between 15 and 20, 35 and 40, 55 and 60). Overall, 5+5+5=15. P=1- 15/60=3/4
Re: A searchlight on top of the watch-tower makes 3 revolutions   [#permalink] 04 Feb 2018, 08:35

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A searchlight on top of the watch-tower makes 3 revolutions

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