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A red light flashes 3 times per minute and a green light flashes 5 tim

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Bunuel
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A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   03 Dec 2020, 01:15
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A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. If both lights start flashing at the same time, how many times do they flash together in each hour?

A. 29

B. 30

C. 31

D. 32

E. 33
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B
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samubel
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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   03 Dec 2020, 19:02
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I think it's 31, option C because you have to add the first time they flash together at t=0
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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   28 Dec 2020, 05:34
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samubel wrote:
I think it's 31, option C because you have to add the first time they flash together at t=0




It's a very common mistake to add the 1st flash. that is at the 0 second instant. the first flash is after the first on at t=0. at t = 120, we get the 30th flash. And it is every hour, which is 30 every hour and not 31.

Hope this helps

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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   03 Dec 2020, 04:32
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Bunuel wrote:
A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. If both lights start flashing at the same time, how many times do they flash together in each hour?

A. 29

B. 30

C. 31

D. 32

E. 33



The red light flashes every 60/3 = 20 seconds and the green light flashes every 120 / 5 = 24 seconds

LCM of 20 and 24 = 120 second = 2 minutes

So after the first flash, they will flash together 60/2 = 30 times



Option B

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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   06 Dec 2020, 09:49
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Bunuel wrote:
A red light flashes 3 times per minute and a green light flashes 5 times in two minutes at regular intervals. If both lights start flashing at the same time, how many times do they flash together in each hour?

A. 29

B. 30

C. 31

D. 32

E. 33

Solution:

We see that the red light flashes 6 times in 2 minutes whereas the green light flashes 5 times in 2 minutes. Therefore, every 2 minutes, they will flash at the same time. Since in an hour (or 60-minute period), there are 30 two-minute intervals, the two lights flash together 30 + 1 = 31 times (recall that if we mark each time they flash together, the number of markings is always 1 more than the number of intervals between the markings.)

Answer: C
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abhinavsodha800
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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   28 Dec 2020, 05:19
Hi Bunuel,
What is the correct answer is it 30 or 31 ? Since they are going to flash every two min do we need to consider interval as well ?
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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink] New post   11 Oct 2023, 16:30
Don't know if my reasoning is correct here, but I got the correct answer

the red light will flash 3*60 = 180 times in an hour
green light will flash 5*30 = 150 times in an hour

180 = 2*2*3*3*5
150 = 2*3*5*5

HCF of 180 and 150 is then 2*3*5 = 30

ANS B

But is the reasoning right ?
thanks for your help
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Re: A red light flashes 3 times per minute and a green light flashes 5 tim [#permalink]
11 Oct 2023, 16:30
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