- A11
- B10
- C12
- D6
Step-by-Step Solution
LCM(40, 60, 72) = 360 s = 6 minutes. Joint flashes come at 6:00, 6:06, …, 7:00: 60 ÷ 6 + 1 = 11 times.
Answer: A) 11
Three lighthouses flash every 40 s, 60 s and 72 s. They flash together at 6:00 PM. How many times will they flash together from 6:00 PM to 7:00 PM, counting both ends?
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