general ability
posted by cindy on .
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

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