Light Houses: at what time, on what day, will all three flas

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LaryssaLacerda

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Three lighthouses on three nearby islands flash their lights every morning. On one island, the light flashes every 45 seconds. The lighthouse on the second island has a light that flashes every 2 minutes. The third lighthouse flashes a light every 1 and 1/4 minutes. At 6PM on Monday, all three lights flash simultaneously. At what time, on what day, will all three lights flash together again? (hint: least common multiples)

WORK DONE :

45 seconds least common multiple: 5*3*3
120 seconds least common multiple: 5*2*2*3
75 seconds least common multiple: 3*5*5

I have no idea on what I should do next..

Can you please help me..? Thank-you so much
 
Convert all to seconds. What's the LCM of 45, 75, 120?. That will be in seconds. Convert to minutes and add to the time given.
 
Thank-you so much.. I really did learn it.. That website is very good to learn.. and very easy to understand..

I found out that the LCM of 45, 120 and 75 is 1800.
 
Very good. Now, that's in seconds. Convert to minutes.
 
1800 is 30 minutes.. so..

the first lighthouse= 30 minutes and 45 seconds
the second lighthouse= 32 minutes
the third lighthouse= 31 minutes and 25 seconds.
 
there is still one more question.. At what time, on what day, will all three lights flash together again? How do I answer that?
 
LaryssaLacerda said:
1800 is 30 minutes.. so..

the first lighthouse= 30 minutes and 45 seconds
the second lighthouse= 32 minutes
the third lighthouse= 31 minutes and 25 seconds.
Click to expand...

What is all this about?.

:shock: For heaven's sake, 30 minutes is the answer. Add that to the time given.
 
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