Question

Two light houses flash their lights every 30 seconds and 40 seconds respectively. given that they last flashed together at 10:40

Answer 1

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

Answer 2

Find the common muliple 120seconds is 2mins herefors 10.42