Four bells begin to toll together and toll respectively at intervals of 6, 7, 8 and 9 seconds. In 1.54 hours, how many times do they toll together and in what interval (seconds)?

Answer: Option C

Interval after which the bells will toll together = (LCM of 6, 7, 8, 9) sec = 504 sec.
In 1.54 hours, they will toll together
$$\begin{array}{rl}& \text{ = }\left[\left(\frac{1.54\times 60\times 60}{504}\right)+1\right]\\ & \text{ = 12 times}\end{array}$$