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Question

Three lights flash at intervals of 5, 8 and 10 seconds respectively.

If they are all switched on at the same time, how soon after will they flash together again?

A
20 seconds
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B
40 seconds
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C
60 seconds
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D
80 seconds
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Solution

The correct option is B 40 seconds
To find when the three lights will flash together again, we need to find the least common multiple (LCM) of the numbers 5, 8 and 10.

Multiples of 5 = 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, ...

Multiples of 8 = 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, ...

Multiples of 10 = 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, ...

40 is the smallest multiple common to 5, 8 and 10.

The three lights will flash together again after 40 seconds.

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