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Verified Answer :

find the common muliple 120seconds is 2mins herefors 10.42

Apocrophon

If they flash at 30 and 40 seconds, then a diagram showing their timing helps

30, 60, 90, 120
40, 80, 120

So after 120 seconds, both lighthouses flash at the same time
120 sec = 2 minutes

So the answer is 10:42

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