HEWGE blind spot about percentages - how did they solve this!?!

ulrichburke

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Jun 10, 2016
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Dear Anyone.

So I wanted to find out what percentage Number A was of Number B - because I can never remember how to do that one - and I found a site that said if I typed the numbers into boxes it would explain step-by-step how it reached its answer.

Now I'm NOT gainsaying the answer itself - but it suddenly comes up with a great long fraction from SOMEWHERE and I can't for the life of me see where it got it from!!

Please be gentle with the answer, I know I should see (and remember) how to do this one but I've never understood it.... I'm going to copy and paste their entire solution here and if anyone could tell me where they got this whacking great decimal from.....


1. We assume, that the number 61000 is 100% - because it's the output value of the task.
2. We assume, that x is the value we are looking for.
3. If 100% equals 61000, so we can write it down as 100%=61000.
4. We know, that x% equals 7100 of the output value, so we can write it down as x%=7100.
5. Now we have two simple equations:
1) 100%=61000
2) x%=7100
where left sides of both of them have the same units, and both right sides have the same units, so we can do something like that:
100%/x%=61000/7100
6. Now we just have to solve the simple equation, and we will get the solution we are looking for.

7. Solution for 7100 is what percent of 61000

100%/x%=61000/7100
(100/x)*x=(61000/7100)*x - we multiply both sides of the equation by x
100=8.5915492957746*x - we divide both sides of the equation by (8.5915492957746) to get x
100/8.5915492957746=x
11.639344262295=x
x=11.639344262295

now we have:
7100 is 11.639344262295% of 61000

OK, the answer's correct - but where the heck did that 8.5915492957746 come from!?! I've read and re-read that thing and that number just pops out of nowhere as far as I'm concerned. I'm fine up until the first line of the arithmetic of Part 7, from then on it's written in Sanskrit to me!! OK multiplying both sides of a number by another number keeps the relationship the same between the numbers, I get that. (I THINK!) But I thought, logically, percentages were more about division? And how the heck can you multiply both sides of the equation by X BEFORE you know what X IS?? They don't find out what X IS until the NEXT step when they use that number they pluck out of the air from somewhere.

Oh, mummy!

Hope someone understands this out there, cos I sure as heck don't.

Yours hopefully

Chris.
 
You should not and most importantly not try to understand what was said. It is complete nonsense.

% means to divide by 100. So 100% = 100/100 = 1 NOT 61000.

They say that x% = 7100. That is x/100 = 7100 or x = 710,000

You want to know what percent of number A is number B. I will let A represent numberA and B represent number B.

What means x, Of means * (times), % means /100 and is means = (equal)

What percent of A is B
(x/100)*A = B

Multiply by sides by 100/A we get x = 100B/A = 61000*100/7100
 
I wouldn't say that what the site said is complete nonsense, but it is not mathematics! It is a naive way to build the proportion 100/x=61000/7100, which is valid, and from there the work is correct, if not efficient. They simply divides 61000 by 7100 and got 8.5915492957746... .

It can be done far more efficiently. At the least, I would have written the proportion upside-down, as x/100 = 7100/61000, so that the desired quantity is on the top. Then all you need to do is to divide 7100 by 61000, and then multiply by 100.

I would not use that site as a source in the future, if I were you. In fact, once you saw that you couldn't follow what they said, you should have just moved on to another site to learn about this! It isn't your fault that you didn't understand.
 
The reason I get so upset with non mathematical math is that it removes one of the most basic principles of mathematics--the equality sign.

Students actually do not have a full understanding of equality and this is not good at all. I was shocked when I wrote the equation 3x+4 = 10 on the board and I asked my students if they could tell me the exact value that 3x+ 4 and they could not. This really concerned me.
 
Thank you both very much for your help, NOW, thanks to you two, I understand it (Yup - dumbass male alert!)

Dr. Peterson, I really just thought I was being incredibly dumbass by not understanding it and I was sitting drinking tea and looking at it and looking at it and I never did spot where that fraction came from (decimal fraction!) Didn't think of moving on because I thought if I didn't understand it there, I wouldn't understand it anywhere else either.

Thankyou both very much for your help. I know not to move on from this site the next time I get stuck on something mathematically dumbass!

Yours respectfully

Chris.
 
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