TWO LIGHT HOUSES CAN BE SEEN FROM TOP OF A HILL .THE FIRST FLASHES ONCE EVERY 8 SECONDC AND OTHER FLASHES ONCE EVERY 15 SECONDS.IF THEY FLASH AT THE SAME TIME ,HOW LONG WILL IT BE UNTIL THEY FLASH AT THE SAME TIME AGAIN
the LCM of 8 and 15 is 8*15 = 120
thus they flash after 120 seconds at the same time.
120 second = 120/60 = 2min.
thus after 2 min both light house flash at the same time.
hope this helps you.
cheers!!