Homework Help: general ability

Posted by cindy on Tuesday, February 19, 2008 at 5:04am.

light X flashes every 3 minutes. Light Y flashes every 5 minutes. Light Z flashes every 7 minutes. At exactly 7:28 AM they all flash together. At what time will they do so again?

a) 9:13
b) 9:15
c) 9:00
d) 9:28

general ability - Reiny, Tuesday, February 19, 2008 at 8:55am
The next time must be a multiple of 3, 5 and 7.
In other words they will flash together 05 minutes from now
or 7 hours, 28 minutes + 105 minutes
= 7 hours, 133 minutes
= 7 hours + 2 hours + 13 minutes

That would make it 9:13

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