Two lighthouses flash every 40 seconds and every 50 seconds. They flash together at 9:00 pm. After how long will they next flash together?

1 minute 30 secondsCorrect answerYour answer2 minutesCorrect answerYour answer3 minutes 20 secondsCorrect answerYour answer33 minutes 20 secondsCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer C

Correct answer: C

They flash together after $\operatorname{LCM}(40, 50) = 200$ seconds.
200 $s = 3$ minutes 20 seconds.

Worth noting

33 min 20 s is the product $40 \times 50 = 2{,}000$ s; 1 min 30 s adds the intervals.

Concept: LCM for repeating signals
Typical timeTypical: 40 s
Your timeYours: —

The rule this testsHCF and LCM word problems

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

Two lighthouses flash every 40 seconds and every 50 seconds. They flash together at 9:00 pm. After how long will they next flash together?

Explanation