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Three bells toll at intervals of $9$, $12$ and $15$ minutes respectively. If they start tolling together, after what time will they next toll together?
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$$\begin{aligned}& 9 = 3^2 \\ & 12 = 2^2 \times 3 \\ & 15 = 3 \times 5 \\ & L.C.M = 2^2 \times 3^2 \times 5 = 180 \\ & \text{Three bells will toll together after } 180 \text{ min.}\end{aligned}$$