Question : Five bells commence tolling together and toll at intervals of 5, 10, 15, 20, and 25 seconds respectively. In 1 hour 20 minutes, how many times do they toll together?
Option 1: 20
Option 2: 15
Option 3: 14
Option 4: 17
Correct Answer: 17
Solution :
Time after which the 5 bells will toll together = LCM of 5, 10, 15, 20, and 25
= 5 × 5 × 3 × 2 × 2
= 300 seconds
= 5 minutes
Total time = 1 hour 20 minutes
= 60 + 20
= 80 minutes
Number of times the bells toll together
= $\frac{80}{5}$ + 1
= 16 + 1
= 17
Hence, the correct answer is 17.
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