Four sirens at a Nagpur factory sound every 10, 15, 25 and 30 minutes. All four sound together at 6:00 am.
How many MORE times will all four sound together before 6:00 pm the same day?

LCM(10, 15, 25, 30) = $2\times 3\times 5^2=150$ minutes = 2 h 30 min. From 6:00 am they next sound together at 8:30 am, 11:00 am, 1:30 pm and 4:00 pm - that is 4 more times, since the following one falls at 6:30 pm. Answering 5 counts the 6:00 am start as well; 24 divides the 12 hours by the longest interval, 30 minutes.