Three buses leave a depot every 15, 20 and 25 minutes. They leave together at 6:00 am. How many times will they leave together from 6:00 am to 9:00 pm, counting both 6:00 am and 9:00 pm?

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CorrectNot quiteAnswer B

Correct answer: B

They leave together every $\operatorname{LCM}(15, 20, 25) = 300$ minutes = 5 hours.
Joint departures: 6:00 am, 11:00 am, 4:00 pm, 9:00 pm.
That is 4 times.

Worth noting

15 hours ÷ 5 hours = 3 counts the gaps, not the departures; add 1 because both ends are included.

Ten questions from HCF & LCM, at your own pace.

Explanation