Six bells start ringing simultaneously and they ring at intervals of 4, 8, 10, 12, 18 and 20 seconds respectively. Once the ringing starts, how many times will the bells ring together in next 30 minutes?

Correct option is D

Given
Intervals: 4, 8, 10, 12, 18, 20 seconds.
Time duration: 30 minutes.

Solution
Step 1: Find LCM of intervals.
LCM(4, 8, 10, 12, 18, 20)
LCM(8, 18, 20) = 360
360 is divisible by 4, 10, 12.
So, LCM = 360 seconds.
360 seconds = $$\frac{360}{60}$$​= 6 minutes.

Step 2: Calculate number of times they ring together.
The bells ring together every 6 minutes.
In 30 minutes, they ring together = $$\frac{30}{6}$$​ = 5 times.
(Note: The question asks for the "next" 30 minutes, usually excluding the start.)

Final Answer
So the correct answer is (d)