Question

Four bells ring for every 30 minutes, 1 hour ,1/2hour ,1 3/4hour .if all bells rang at 12 noon today .when will they ring together again

Answer 1

they ring at 30 mins, 60 mins, 30 mins, 105 mins.
Thus LCM of the above values is the first common multiple of them.
LCM = 420 = 7 HRS
thus next time is 7 pm