Three bells ring at intervals of 4, 9 and 15 seconds. If they ring together now, after how many seconds will they next ring together?

💡 Solution & Explanation

The bells ring together again after a time that is a multiple of all three intervals, and the first such time is the LCM. LCM of 4 and 9 is 36, and LCM of 36 and 15 is 180. So the bells ring together again after 180 seconds. Adding the intervals or multiplying them gives a common time, but not the first one.