correct answer is: a) 2 minutes 6 seconds

Explanation

Five bells begin to toll at the intervals of 1.2,1.5,1.75,1.8,2.1 sec respectively.
Let,
1.2 sec = 120 sec
1.5 sec = 150 sec
1.75 sec = 175 sec
1.8 sec = 180 sec
2.1 sec = 210 sec
To, find the required time when the bells toll together again, we need to find the LCM of the given intervals.
So, the LCM of 120,150,175,180 and 210 sec is:
\rightarrow5\times3\times2\times2\times7\times5\times2\times3=12600
So that, LCM of 1.2 sec, 1.5 sec, 1.75 sec, 1.8 sec and 2.1 sec is =126 sec.
\therefore The bells will toll together after 126 sec or 2 minutes 6 seconds.
Ans: After 2 minutes 6 seconds the five bells will toll together.

Other ways to ask this same math question are:

1. If five bells toll together initially and then toll at intervals of 1.2 seconds, 1.5 seconds, 1.75 seconds, 1.8 seconds, and 2.1 seconds respectively, when will they simultaneously toll again?
2. Five bells toll at the same time initially, and subsequently begin tolling at intervals of 1.2 seconds, 1.5 seconds, 1.75 seconds, 1.8 seconds, and 2.1 seconds respectively. When will they toll simultaneously once more?
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5. Five bells toll simultaneously and then begin tolling at intervals of 1.2 seconds, 1.5 seconds, 1.75 seconds, 1.8 seconds, and 2.1 seconds respectively. When will they simultaneously toll together again?