Shortcut Trick
The time when all bells ring together again is the LCM of their ringing intervals.
Intervals: 18, 24, and 30 seconds.
LCM(18, 24, 30) = 360 seconds.
Time in minutes = 360 ÷ 60 = 6 min.
∴ The correct answer is 6 min.
Alternate Method
Given:
Interval of Bell 1 = 18 seconds
Interval of Bell 2 = 24 seconds
Interval of Bell 3 = 30 seconds
Starting time = 10:00:00 AM
Calculations:
⇒ To find the time when they ring together again, calculate the Least Common Multiple (LCM).
⇒ Prime factorization of 18 = 2 × 3 × 3 = 21 × 32
⇒ Prime factorization of 24 = 2 × 2 × 2 × 3 = 23 × 11
⇒ Prime factorization of 30 = 2 × 3 × 5 = 21 × 31 × 51
⇒ LCM = (Highest power of each prime factor) = 23 × 32 × 51
⇒ LCM = 8 × 9 × 5 = 360 seconds
⇒ Convert seconds to minutes: Time = 360 ÷ 60
⇒ Time = 6 minutes
∴ The correct answer is 6 min.
Additional Information
LCM and Periodic Events
For any events occurring at regular intervals (like bells, traffic lights, or runners), the first time they coincide after starting together is the LCM of their individual time periods.
Relationship between HCF and LCM
For two numbers a and b, the product of the numbers is equal to the product of their HCF and LCM: HCF(a, b) × LCM(a, b) = a × b.
Time Conversion Rules
To convert seconds to minutes, divide by 60. To convert minutes to hours, divide by 60. To convert hours to seconds, multiply by 3600 (60 × 60).