Six bells start ringing together and ring at intervals of 4, 8, 10, 12, 15 and 20 seconds respectively. how many times will they ring together in 60 minutes ?

Answer: Option A

LCM of 4, 8, 10, 12, 15 and 20 = 120
120 seconds = 2 minutes
Hence, all the six bells will ring together in every 2 minutes
Hence, number of times they will ring together in 60 minutes
= $$1+\frac{60}{2}$$
= 31