Answer :
If both lighthouses A and B sound their foghorns at the same time, it will be after 30 hours before both foghorns sound at the same time again.
- Since lighthouse A sounds it's foghorn every 3hours, and lighthouse B sounds it's foghorn every 10 hours.
To determine the next time when both foghorns sound at the same time;
We need to determine the lowest common multiple (LCM) of 3 and 10.
- The lowest common multiple of 3 and 10 is therefore 30.
As such, the foghorns sound at the same time after 30 hours of when the foghorns sounded the same time.
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