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30 minutes             = 1800=  180=  90=  30=  15=       15
LCM of the numbers= 120 =   12=   6=    2  =   1=

Answer is= 15

Its confusing!
And also ur answer is wrong the answer is 16times!
First take lcm of 2,4,6,8,10,12=120
therfore toll altogether at same time is 120sec
how many tolls in 30 minutes=1800sec=?

solution=1800*1/120=15 times
but in the question
six bells commence tolling together(first toll) and tolls at intervals of 2,4,6,8,10,12,
beginning itself tolling we should consider that toll also
therfore 15 times+1 time=16 times