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**noorey711****Member**- Registered: 2005-12-12
- Posts: 2

Robin Reed reported that the coldest day on record for his town was five times colder than

yesterday's temprature, -4°C. What was the temprature of the coldest day in record in

Robin Reed's town?

do I have to divid -4÷5= [-4]÷[5]= 4÷5= -0.8

is this right?

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**justlookingforthemoment****Moderator**- Registered: 2005-05-26
- Posts: 2,161

You were on the right track, but instead of dividing, you should multiply.

-4 x 5 = -20°C

So, the coldest day was -20°C. Brr!

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**noorey711****Member**- Registered: 2005-12-12
- Posts: 2

oh..

Thank you!!!

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,684

Cool optical illusion !

(The website has more, but also LOTS OF ADS)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**justlookingforthemoment****Moderator**- Registered: 2005-05-26
- Posts: 2,161

What are you meant to do?

I've been staring at the middle and the flashing appears to be changing speeds, so I presume that's what's meant to happen.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,684

I get a phantom bright dot ... the eye over-compensates for the missing dot I think.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**mathsyperson****Moderator**- Registered: 2005-06-22
- Posts: 4,900

I get what MathsIsFun gets. Nothing at first, then a bright white dot where there should be nothing. It also has a hint of green for no apparent reason. Did anyone else get that or are my eyes weird?

The question raises an interesting point. How can you calculate how much is twice as cold?

Doing the same thing with °C, °F and K would return different answers. Also, what is twice as cold as 0°? What's the difference between twice as cold and twice as hot?

Why did the vector cross the road?

It wanted to be normal.

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,684

That is one of the best optical illusions ever!

Quite right about "twice as cold", it really makes no sense for two reasons:

1. Temperature is just an arbitrary scale. To have it make sense, you would need to measure the amount of heat energy. Then you could say "this has twice as much heat".

2. Cold is just the absence of heat, so saying "twice as cold" is like saying "twice as poor".

Having said that, -20°C would still be the "commonly accepted" answer

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**justlookingforthemoment****Moderator**- Registered: 2005-05-26
- Posts: 2,161

These optical illusions are great!

I see what you mean about temperature. It's kind of like saying 'I'm three times as short as you'. How can you be twice as short? It makes more sense when you say 'I'm a third as tall'.

I spent a long time playing with many more puzzling ones here:

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,684

That is a great website ... what a collection.

I had to say to myself "later!" or I would get nothing done today.

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