Math Is Fun Forum

Marymo

Member

Registered: 2015-11-18

Posts: 2

Is a decimal a percentage "in the US"?

Hi everyone,

this morning I became very confuse, together with  my son. We moved to CA 8 months ago and love it, kids are doing great in school. However, in his 6th grade middle school my son is currently learning strange things in math - in my eyes. Is there anybody who could please add more US perspective to my experience?
As they are working with fractions, decimals and percentage, they make a lot of use of changing between theses kind of expressions. In his assessment I just saw the task "change 1.074 to percent" and the right answer would have been 107.4%. This is very awkward to me! Any number -  being integer, natural, rational or even real can be a percentage OF ANOTHER GIVEN AMOUNT but not in itself. And there was no base amount given with the task. The next task was then "change 578.3% to a decimal" and the answer of 5.783 was expected.
To say the truth, I am flattened by this teaching and my son is now quite confused ...

Looking forward to insights!!

David

Member

From: Bumpkinland

Registered: 2014-04-23

Posts: 3,164

Re: Is a decimal a percentage "in the US"?

0.50 is 50% | 0.50 x 100% = 50%
1 is 100% | 1 x 100% = 100%

5 is 500% | 5 x 100% = 500% 

Common Core is to blame. The new standards does not explain how it's done...

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His dominion is an everlasting dominion, Which shall not pass away, And His kingdom the one Which shall not be destroyed.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,163

Re: Is a decimal a percentage "in the US"?

hi Marymo

Welcome to the forum.  Take a look at this page:

http://www.mathsisfun.com/decimal-fract … ntage.html

I think this will help to clear up your doubts.  And it's exactly the same in the UK.  Fractions, decimals and percentages are three different ways of expressing a number.  Get your son to try out the interactive conversion diagram.  It's a great way of showing what is happening.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Marymo

Member

Registered: 2015-11-18

Posts: 2

Re: Is a decimal a percentage "in the US"?

Hi and thanks for the prompt replies! I saw this kind of conversions in my son's textbook. However, still I cannot believe this is really being taught!

0.5 is not equal to 50% just if you assume that it is 0.5 of 1
> 0.5 can be 20% in case of being 0.5 of 2.5
> 0.5 can be any % depending on the base
and exactly this is the bad teaching. A number being integer or decimal is NEVER a portion except you express it explicitly in such context.

This is horrible mathematics for me ...

sorry ;-)

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Is a decimal a percentage "in the US"?

Hi;

I was taught that very thing in school close to 100 years ago and agree with those answers. 1 is the implied base as you say.

If someone says I have one quarter of all the money, that means that I have 25 % of all the money or .25. It does not matter how much money there is, I have 25% of it and I am going to Aruba.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,163

Re: Is a decimal a percentage "in the US"?

hi Marymo

This makes sense and it appears to be the way you have been taught about percentages.  So I can understand that it is a bit of a shock to  discover that others are being taught differently.

I did an internet search for a definition of percentage.  You can try this yourself.  I followed up just a few and found they were all in agreement.  Per cent means out of 100.  That's where the word comes from.  So what you are calling the 'base' is always 100.  You don't get to choose something else as the base.  Percent is just a way of expressing a fraction is a stylised way.  It makes comparison easier.

eg.

John got 65/80 in a math exam and 35/40 in a science exam.  In which did he do better ?

Note that for example

So a percentage and a fraction are interchangeable and as all fractions can be written as decimals, that means decimals and percentages are also interchangeable.

It's a lot easier to change between a decimal and a percentage because you only have to move the decimal point two places.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob