Difficulty: Tricky

Last week, there was an earthquake near Mount Buller in Victoria.

The shaking reached central Melbourne around 9:16:30, and Canberra at 9:17:30.

It’s 120 kilometres from the earthquake epicentre to central Melbourne, and 360 kilometres from the epicentre to Canberra. From the epicentre to Sydney, it’s 600 kilometres.

What time did the shaking get to Sydney?

Scroll down or click for a hint, or the answer!


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Brainteaser hint

If you’re having difficulty understanding this brainteaser, imagine that the vibrations headed out from the epicentre in all directions right when the earthquake first hit.

A good first step would be to work out the speed of the vibrations. You might be able to use this to work out when the earthquake happened, or you might be able to find the solution without doing that at all!

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Brainteaser answer

We don’t know exactly when the earthquake happened, but we know the vibrations had travelled 120 kilometres by 9:16:30. And that includes the vibrations headed towards Canberra. One minute later at 9:17:30, they reached Canberra. In that minute they travelled 360 – 120 = 240 kilometres.

At 9:17:30, the vibrations had travelled 360 kilometres towards Sydney. They still had 600 – 360 = 240 kilometres to go. We already know that they’re travelling at 240 kilometres per minute, so it should take one more minute. So the vibrations arrived at 9:18:30.

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