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# Four flash lights begin to flash light together and then at

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Senior Manager
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Four flash lights begin to flash light together and then at [#permalink]

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01 Mar 2004, 14:08
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Four flash lights begin to flash light together and then at intervals of 6,7,8 and 9 seconds respectively. How many times will the flash lights be on together in a duration of 2 hrs?

14
18
144
504
604
_________________

Pls include reasoning along with all answer posts.
****GMAT Loco****
Este examen me conduce jodiendo loco

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01 Mar 2004, 14:37
Ans: A

Total sec in 2 hours = 2 * 60 * 60
Lights flash togather at an interval of 504 sec.
Divide and we get 14.
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05 Mar 2004, 14:59
Let me explain it

Let us say one light flashes at 2 seconds and another at 5 seconds then they both coincide at 10 seconds. In 20 seconds they coincide 2 times in 40 seconds they do 4 times.
They way to solve this is take LCm of 2 and 5 you get 10
in 40 seconds they coincide 40/10 = 4 times
LCM of ( 6,7,8,9 ) is 504
In 2 hours we have 60 * 60 * 2 seconds
So they flash together 7200/504 times = 14 times.
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