Shortcut Trick
Find the LCM of ringing intervals: LCM(4, 8, 12, 16, 20, 24) = 240 seconds.
Convert interval to minutes: 240 seconds ÷ 60 = 4 minutes.
Number of rings = (Total Time ÷ LCM interval) + 1 (Commencement ring).
Number of rings = (18 ÷ 4) + 1 = 4.5 + 1 → 4 + 1 = 5 times.
∴ The correct answer is 5.
Alternate Method
Given:
Intervals = 4, 8, 12, 16, 20, 24 seconds
Total time = 18 minutes
Formula Used:
Simultaneous ringing interval = LCM of individual intervals.
Total occurrences = [Total Time ÷ LCM] + 1
Calculations:
⇒ LCM of 4, 8, 12, 16, 20, 24:
⇒ 4 = 22
⇒ 8 = 23
⇒ 12 = 22 × 3
⇒ 16 = 24
⇒ 20 = 22 × 5
⇒ 24 = 23 × 3
⇒ LCM = 24 × 3 × 5 = 16 × 15 = 240 seconds
⇒ LCM in minutes = 240 ÷ 60 = 4 minutes
⇒ Number of times in 18 minutes = (18 ÷ 4) + 1
⇒ Number of times = 4.5 + 1 ≈ 4 + 1 = 5
∴ The correct answer is 5.
Additional Information
LCM for Simultaneous Events
When multiple events occur at different fixed intervals, they will happen together at time intervals equal to the Least Common Multiple (LCM) of their individual periods.
Including the Commencement Ring
In problems stating they ring together at the start, always add 1 to the quotient of (Total Time ÷ LCM Interval) to account for the t = 0 occurrence.
Integer Division Rule
If the total time is not perfectly divisible by the LCM, we only consider the integer quotient, as the next simultaneous ring would occur after the given time limit.