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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,558

For example: when you multiply by 100 do you "move the decimal point 2 places to the right" ... or do you "move the number 2 places to the left".

And do you have reasons for your choice?

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

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi MIF;

Never gave it any thought ( authority figures like teachers were obeyed then, hmmm ). We were taught to move the decimal 2 places to the right.

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,820

hi MIF

we too were taught to move the decimal point to the left by 2 places when you multiply by 100,but now when i'm thinking about it ,it may be more intuitive the the number moves to the left by two places.

*Last edited by anonimnystefy (2012-01-23 21:35:09)*

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,726

hi MIF,

My preference is that the columns are fixed with the decimal point at the dividing line between whole numbers and decimals less than one.

So that means, for me, it is the numbers that move.

In the UK at primary level children are taught initially to put column headings: hundreds, tens and units and must keep columns vertically neat, so that adding etc works nicely.

If, say, you want to develop understanding of x ten you can then proceed like this.

eg 3 x 10. Put tens and units column headings. Enter a 3 in the units column. Then another underneath, then another and so on until you have ten of them. Add them up. Hopefully get 30. Enter this result with a 3 in the tens column and a zero in the units column. It seems basic but, if children follow these rules consistently, then doing 0.3 x 10 is just the same, except you put the .3s in the tenths column.

See picture.

Bob

ps.

authority figures like teachers

Who are they then?

*Last edited by bob bundy (2012-01-09 11:51:16)*

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

Hi MIF;

What prompted that question? Do you know something?

Hi Bob;

The authority figures were anyone who was older than I. Since I was about 7 that meant the entire world was to be treated with respect. I agree, an outdated concept. Even I, the older I got there were obviously less people to treat that way. When I reached my present age, only George Burns and Bob Hope needed to be treated that way. Now they are dead...

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

**Online**

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,558

I use "move the decimal" with percentages, but I get the odd email "we don't do it like that". The problem is that it is taught in different ways around the world.

I take the point (sic) about a fixed decimal position, but still feel that in practice we mentally move the decimal when multiplying by 10.

(bobbym: you can still look up to Mickey Rooney)

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

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 92,225

I forgot about him.

Wait! Remember the old style calculators with the LED ( red ) digits. If you put a number in there and multiply by 100, using a strobe light ( at a precise frequency of 1.2387 khz ) you can see the decimal point actually crawling to the right. Sort of like those old follow the bouncing ball sing alongs.

**In mathematics, you don't understand things. You just get used to them.**

**I agree with you regarding the satisfaction and importance of actually computing some numbers. I can't tell you how often I see time and money wasted because someone didn't bother to run the numbers.**

**Online**

**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,585

If you look thru a red password games red plastic see-thru filter at the same time, you might see even less, or perhaps depending on the thinness of the cellophane, it might actually act as a pass-filter. Assume the cellophane is only 4 molecules thick and red in color. It appears red when placed on a sheet of white paper. But this non-opaque planar object not only reflects, but it refracts and lets light straight thru too. The whole color filter thing is very curious, and not well explained to me, because how do you know just because you can see something thru it, that it is a filter? You can presume it is a filter, and then go from there, but what if it is something else, and we are making assumptions about what it does before we even know if the colors we see are a wave or a particle or both at the same time? Anyway, I like to move the number left and right instead of the decimal today, since it seems so cool!!

**igloo** **myrtilles** **fourmis**

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**Alex23****Member**- Registered: 2012-01-31
- Posts: 19

Both is the correct answer.

But intuitively moving the number to the left is best. That is because the decimal point is fixed, it separates non-negative powers of ten from negative ones.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,820

hi Alex23

what would you say, does the decimal point move or does the duodecimal point move?

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**Alex23****Member**- Registered: 2012-01-31
- Posts: 19

We move them in practice , either one depending on the representation base used, because it's easier to "cut&paste" a dot rather than all digits, but of course the point is fixed. It's like the birds with their heads when walking!

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,820

i see that you didn't get it.the pun was that the decimal point is decimal in both decimal and duodecimal system.

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**Alex23****Member**- Registered: 2012-01-31
- Posts: 19

Oh yeah, it's just that decimal comes form deca = ten in greek. So it would make sense if we had a duodecimal system it would be called duodecimal point from duo + deca = twelve in greek.

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,820

that's my whole 'point',but i'm not very good at making puns as you might see.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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