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

A 15
B 16
C 30
D 31

Correct Answer: 31

LCM of 4, 8, 10, 12, 15 and 20 = 120 120 seconds = 2 minutes Hence all the bells will ring together in every 2 minutes Hence, number of times they will ring together in 60 minutes = 1 + <math xmlns = "http://www.w3.org/1998/Math/MathML"><mfrac><mn>60</mn><mn>2</mn></mfrac></math> = 31
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