three bells at a factory ring at intervals of 15 minutes,20 minutes and 30 minutes to make shifts of different department if they first ring together at 08:00 am when do they ring together again ?

15 = 3 * 5

20 = 2 * 2 * 5
30 = 3 * 2 * 5
so the least common multiple needs two 2s , one 3 and one 5
2*2*3*5 = 60 minutes
try every hour :)