Decreasing by percentage

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John Mortal

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So, my problem is I don't understand how 0.45 becomes 0.55. I misunderstand this moment when 0.45 turns into 0.55. What is a reason of it? ? Can you explain me it, please? Thank you!
 

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John Mortal said:
So, my problem is I don't understand how 0.45 becomes 0.55. I misunderstand this moment when 0.45 turns into 0.55. What is a reason of it? ? Can you explain me it, please? Thank you!
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Technically - all the posted answers are incorrect.

But I am from India and and have been trained to read mind. So here is a hint:

1 - 0.45 = 0.55
 
John Mortal said:
So, my problem is I don't understand how 0.45 becomes 0.55. I misunderstand this moment when 0.45 turns into 0.55. What is a reason of it? ? Can you explain me it, please? Thank you!
Click to expand...
If you are subtracting 45% from something then you will be left with 55% of the original quantity,
ie: 100% - 45% = 55% (1 - 0.45 = 0.55).​

Therefore, the "quick" way to calculate 90 - 45% is to multiply 90 by 0.55 thus finding out directly what 55% of 90 is, ie: what you are left with after removing 45% of it.
John Mortal said:
Thank you! But what do you mean with 'technically all the posted answers are incorrect'?
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Just ignore the purists.
 
The Highlander said:
If you are subtracting 45% from something then you will be left with 55% of the original quantity,
ie: 100% - 45% = 55% (1 - 0.45 = 0.55).​

Therefore, the "quick" way to calculate 90 - 45% is to multiply 90 by 0.55 thus finding out directly what 55% of 90 is, ie: what you are left with after removing 45% of it.

Just ignore the purists.
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Okay. Thanks! So, 90 - 45% = 90 * 0.55 = 49.5 (Actually, 49.5 is 55% of 90). Is it correct? Now, I know, but if it is 45% of 90, next 90 - 45% from 90 = n. Does he mean you need to know from where a percent is? Is it 45% of 90 or maybe 22? Just my maths skill isn't the best skill of my skillset ? . I'm sorry.
 
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The Highlander said:
Just ignore the purists.
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I am far from a purist, but do insist that 90 - 45% is meaningless--unless you want to say that 90 -45% = 90-.45 = 89.55.

To the op, from 90, you do not subtract 45%. What you do is 90 - 45% of 90= (1-.45)*90.

It is true that nothing on that page is true.
 
Steven G said:
I am far from a purist, but do insist that 90 - 45% is meaningless--unless you want to say that 90 -45% = 90-.45 = 89.55.

To the op, from 90, you do not subtract 45%. What you do is 90 - 45% of 90= (1-.45)*90.

It is true that nothing on that page is true.
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Technically, it isn't meaningless. It's a matter of context.

In mathematics, 90-45% would mean 90-0.45, as you suggest ... except that in mathematics we never write that.

In business, it is my understanding that 90-45% is commonly taken to mean 90-45% of 90. They need to write such things efficiently, so they do. And they know perfectly well what they mean.

In fact, basic calculators take it that way; see this explanation from Microsoft of the fact that their calculator, in standard mode, does as such people expect:

How does the calculator percent key work? - The Old New Thing

The same way it does on a cheap calculator.
devblogs.microsoft.com

The Windows calculator percent sign works the same way as those cheap pocket calculators (which are often called four-function calculators even though they have around six function nowadays). What you first have to understand is that the percent key on those pocket calculators was not designed for mathematicians and engineers. It was designed for your everyday person doing some simple calculations. Therefore, the behavior of the key to you, an engineer, seems bizarrely counter-intuitive and even buggy. But to an everyday person, it makes perfect sense. Or at least that’s the theory.​

I wish it weren't that way. When I've seen an algebraic calculator that has a % button, it has bothered me, because the button on such a calculator tends to work as we would expect, not as basic calculators do; I've sometimes had to warn students about that when they move up from a non-algebra based "business math" course to anything with algebra and get a better calculator. At best, they're inconsistent; the non-mathematician notation doesn't extend well to larger calculations.

So within the mathematical world, I'd be a purist; but I am aware of the reality of the non-mathematical world, just as I am aware of non-standard dialects of English and try not to put down those speakers for using grammar that is wrong in my world.

I wouldn't advise ignoring purists; but I would advise purists to be a little understanding.
 
Dr.Peterson said:
What you first have to understand is that the percent key on those pocket calculators was not designed for mathematicians and engineers.
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This seems to hold for most Microsoft products :) Still remember trying to typeset math in MS Word :(
 
Harry_the_cat said:
70 + 30% should be 70 + 30% of 70.

Otherwise it is as Subhotash has explained.

(I think we are some of the purists that The HIghlander told you to ignore!! :unsure:)
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It was merely a suggestion. ?

The OP is perfectly at liberty to have his initial confusion further aggravated by some of the comments posted above! ?

Steven G said:
I am far from a purist, but do insist that 90 - 45% is meaningless--unless you want to say that 90 -45% = 90-.45 = 89.55.

To the op, from 90, you do not subtract 45%. What you do is 90 - 45% of 90= (1-.45)*90.

It is true that nothing on that page is true.
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90 - 45% is perfectly meaningful (to most ordinary folks).

If you went into a shop and saw a sign posted on an item you were keen to get saying: “Massive Clearance Sale; This week only: this Graphing Calculator is $90 – 45% !”, you wouldn’t expect to pay $89.55 for it would you?

You’d be breathlessly stumping up $49.50 for your delightful new toy! ?
 
The Highlander said:
If you went into a shop and saw a sign posted on an item you were keen to get saying: “Massive Clearance Sale; This week only: this Graphing Calculator is $90 – 45% !”, you wouldn’t expect to pay $89.55 for it would you?

You’d be breathlessly stumping up $49.50 for your delightful new toy! ?
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When I go into a store and it says an apple costs .50 cents of cause I know what that means. That doesn't mean it is OK to do. .50cents and 50 cents are different.
 
Steven G said:
When I go into a store and it says an apple costs .50 cents of cause I know what that means. That doesn't mean it is OK to do. .50cents and 50 cents are different.
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Indeed! The whole point is that you "know what that means", despite the Mathematical solecism! ?
 
I admit I had to look that up, although I had a good guess at the meaning.

1665814300281.png

Duz that mean wee don't hav to spell correktly eyether? U no watt eye meen??
 
Harry_the_cat said:
I admit I had to look that up, although I had a good guess at the meaning.

View attachment 34349

Duz that mean wee don't hav to spell correktly eyether? U no watt eye meen??
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My perspective would be that a teacher can simultaneously (kindly) point out mistakes in a student's grammar, word choice, or notation, and also understand what they mean and be able to work with them. We want students to spell correctly, but we can still understand them and answer their real questions (just as I am doing now!).

This applies especially to students whose first language is not English, or whose first language is not mathematics.
 
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