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Old February 28 2013, 08:41 PM   #1
Kenbushway
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Math question

I am trying to get better at math, the highest math I have is Algebra. For some reason I just can't do math. I have this homework helpers book about basic math and pre-algebra to help me understand math.

Its talking about negative integers and distributive property. I was following along just fine until I got to this example it asked me to work out.

25x96

Now I thought I had this, so I went about to do this:

(25x90) + (25x6) to get my answer but then I read how the book does it and I am completely lost. This is how the book did it:

25x96 = 25 x (100-4) = 25x100 + 25 x (-4)
= 2,500 - 100 = 2,400

Where did they get this 100-4, I realize it equals 96 but what? Can anyone explain to me why it went by 100? What am I missing here?
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Old February 28 2013, 08:44 PM   #2
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Re: Math question

that just seems ridiculously overcomplicated
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Old February 28 2013, 08:50 PM   #3
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Re: Math question

Because the exemple illustrates negative integers and distributive property ?
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Old February 28 2013, 09:36 PM   #4
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Re: Math question

yes, precisely. They needed an example for multiplying the opposit of a sum. the way you did was multyplying sums. Nothing wrong with it at all, but not what they wanted to explain.

Actually the 100-4 makes sense. You can multiply by 100 by simply adding two 0es or shifting the decimal point (or in Europe the decimal comma) two digits to the right. This is a mechanical step that doesn't require much thinking and goes quickly.
Then they substract the rest.

Imagine their example as:
there are 25 boxes with 100 cupcakes each. Someone steals 4 cupcakes from every box. How many are left?
See? Now it makes perfect sense (and a belly ache for the thief LOL)
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Old February 28 2013, 10:39 PM   #5
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Re: Math question

They are also demonstrating that numbers can be defined many ways. 96 can be defined as 100-4 (that's where they got it from, to answer your question).

Of course, 96 could also be defined as 90+6 or 8(12), etc.
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Old February 28 2013, 11:18 PM   #6
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Re: Math question

...or 4 x 24, which makes obtaining the answer trivial.
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Old February 28 2013, 11:24 PM   #7
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Re: Math question

Kenbushway wrote: View Post
Where did they get this 100-4, I realize it equals 96 but what? Can anyone explain to me why it went by 100? What am I missing here?
Because it's easier that way. That's how I figured it out in my own head before I looked at the answer in your post. I recognized from experience that 96 is a multiple of 4 (sometimes it's just a matter of practice and experience, which is why they have us memorize multiplication tables -- or at least they did when I was a kid), and when I was trying to remind myself what it factored down to, I realized, "Hey, 96 is only 4 less than 100, and I know that 100 divided by 4 is 25, so 96 divided by 4 must therefore be 24." Which was an easy way of figuring it out. (Otherwise I would've broken it down as 80 + 16, which is (20 + 4) x 4, and that would've given me the answer.)

And that meant that 25 x 96 could also be written as 25 x (4 x 24), and you just move the parentheses and it's (25 x 4) x 24 = 100 x 24 = 2400. It's just an easier and more intuitive route to the solution.

So that's two different points where it was possible to get to 100, and that made it much easier to solve those parts. We use base-10 mathematics because we have ten fingers, so anytime you can rearrange an equation to get a 10 or 100 or the like in there, it makes it easier to solve.
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Old March 1 2013, 01:44 AM   #8
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Re: Math question

Asbo Zaprudder wrote: View Post
...or 4 x 24, which makes obtaining the answer trivial.
The answer's not the point, though. Learning about distributive property is.
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Old March 1 2013, 03:33 AM   #9
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Re: Math question

Shaytan wrote: View Post
Because the exemple illustrates negative integers and distributive property ?
thestrangequark wrote: View Post
Asbo Zaprudder wrote: View Post
...or 4 x 24, which makes obtaining the answer trivial.
did you miss the part where I said I am bad a math, I will try again, I am worst than stupid when its comes to math.

The answer's not the point, though. Learning about distributive property is.
Yes. I have learned recently to not worry so much about finding an answer but learning the process and many different ways to look at it, one of the reasons I bought this book, saw it the reviews that it explained different ways to do math.
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Old March 1 2013, 03:54 AM   #10
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Re: Math question

^Hey, fix that quote, boy! I'm good at math!
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Old March 1 2013, 06:01 AM   #11
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Re: Math question

When I signed up I was told there would be no math. I think it's in the FAQ.

My math score was so low on my SATs, I think they had to go negative.
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Old March 1 2013, 11:16 AM   #12
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Re: Math question

Christopher wrote: View Post
anytime you can rearrange an equation to get a 10 or 100 or the like in there, it makes it easier to solve.

This is very true. There are lots of "tricks" you can use - like knowing the digits of anything divisible by 3 will, themselves, sum up to something divisible by 3.

For example:

1386486

Sum the digits: 36.
Sum the digits again: 9.

So it's divisible by 3.

Similarly anything divisible by 9 can have its digits added up to something divisible by 9. (So 1386486 above is divisible by 9.)

You learn them mostly through experience, but also sometimes through being taught them.
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Old March 1 2013, 12:48 PM   #13
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Re: Math question

Book method makes perfect sense. It's stupid-easy to figure out what 100 x anything is, so you can get to 2500 instantly. Then, since you only needed 96 of them instead of 100, just take 4 back off, so -100.

Breaking it into 90 and 6 doesn't help...
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Old March 1 2013, 01:39 PM   #14
Asbo Zaprudder
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Re: Math question

Thank the Indians for the zero and place-value notation.
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Old March 1 2013, 02:11 PM   #15
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Re: Math question

To the OP...Order of Operations.

Also, buy a calculator.
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