Three bells ring at intervals of 9, 18 and 20 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 9 and 18 is 18, and LCM of 18 and 20 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.