Four bells toll at intervals of 8,9,12and 15 minutes respectively. if they toll together at 3 p. m., when will they toll together next.?

We need to find a number that is in the multiplication table for 8,9,12 and 15.
Lets write down the multiplication table for 15 to a certain way, and see if we recognise those numbers in the other tables (you can write these table down as well but I imagine the number we are looking for is quite far in).

15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195, 210, 225, 240, 255, 270, 285, 300, 315, 330, 345, 360, 375, 390, 405, 420, 435, 450, 465, 480, 510, 525, 540, 555, 570, 585, 600.

Which ones of these numbers can be found in the 12 table?
60, 120, 180, 240, 300, 360, 420, 480, 540, 600.

And out of these, which ones in the 9 table?
180, 360, 540.

And out of these, which ones in the 8 table?
360.

There we go, we found the earliest number in all of these tables. This means that tell will all toll together every 360 minutes, or 6 hours.
If they toll together a 3pm, they will toll together next at 9pm.