# Subtracting Percents / Fractions

• Feb 7th 2010, 03:00 PM
pychon
Subtracting Percents / Fractions
I feel so embarrassed... use to do geom/trig on the fly now I can't do simple 5th grade arithmetic. Trying to figure out:

$\displaystyle 8\frac{1}{4}$ % - $\displaystyle 5\frac{2}{5}$%

Into:
$\displaystyle 8\frac{5}{20}$ % - $\displaystyle 5\frac{8}{20}$%

Into (where does the 25 come from? I see 1 was taken away from the 8, please explain)
$\displaystyle 7\frac{25}{20}$ % - $\displaystyle 5\frac{8}{20}$% = $\displaystyle 2\frac{17}{20}$

Converting to decimal it's:
0.0825 -0.0540 = 0.0285 or $\displaystyle =\frac{285}{10,000}$ simplified... I gave up.
• Feb 7th 2010, 03:04 PM
pickslides
Quote:

Originally Posted by pychon
Trying to figure out:

$\displaystyle 8\frac{1}{4}$ % - $\displaystyle 5\frac{2}{5}$%

is

$\displaystyle \frac{33}{4}$ % - $\displaystyle \frac{27}{5}$%

As % means divide 100

$\displaystyle \frac{33}{400}$ - $\displaystyle \frac{27}{500}$

Now find a common denominator and subtract.
• Feb 7th 2010, 03:13 PM
pychon
I'm not familiar with your way... never made percentages into improper fractions, was taught to rename the fractional parts with the same common denominator. Keep in mind I'm looking for explanations.

The problem is solved... I just need to know where $\displaystyle 7\frac{25}{20}$ came from or an easier way to do these.
• Feb 7th 2010, 03:22 PM
pickslides
Quote:

Originally Posted by pychon
I just need to know where $\displaystyle 7\frac{25}{20}$ came from or an easier way to do these.

I get,

$\displaystyle \frac{33}{4}- \frac{27}{5}$

$\displaystyle \frac{33}{4}\times \frac{5}{5}- \frac{27}{5}\times \frac{4}{4}$

$\displaystyle \frac{165}{20}- \frac{108}{20}$

$\displaystyle \frac{165-108}{20}$

$\displaystyle \frac{57}{20}$
• Feb 7th 2010, 03:34 PM
pychon
Tried to PM, but ridiculous post count rules:

I know you're trying to help. But I need explanations to your solutions... not just how you devised it in your head and displaying the fractional equations.

It's been "years" since I've done any of this stuff... explanations of "how" you worked out the solution would be greatly appreciated!
• Feb 8th 2010, 02:49 AM
HallsofIvy
Quote:

Originally Posted by pychon
I feel so embarrassed... use to do geom/trig on the fly now I can't do simple 5th grade arithmetic. Trying to figure out:

$\displaystyle 8\frac{1}{4}$ % - $\displaystyle 5\frac{2}{5}$%

Into:
$\displaystyle 8\frac{5}{20}$ % - $\displaystyle 5\frac{8}{20}$%

Into (where does the 25 come from? I see 1 was taken away from the 8, please explain)

8= 7+ 1 and $\displaystyle 1= \frac{20}{20}$
$\displaystyle 8\frac{5}{20}= 7+ 1+ \frac{5}{20}= 7+ \frac{20}{20}+ \frac{5}{20}$

Quote:

$\displaystyle 7\frac{25}{20}$ % - $\displaystyle 5\frac{8}{20}$% = $\displaystyle 2\frac{17}{20}$

Converting to decimal it's:
0.0825 -0.0540 = 0.0285 or $\displaystyle =\frac{285}{10,000}$ simplified... I gave up.
• Feb 8th 2010, 05:48 AM
pychon
I'm sorry, but that doesn't explain anything to me. Like I said... would be nice for explanations, not worked out solutions.
• Feb 8th 2010, 01:09 PM
Quacky
I thought the explanations were very clear. If you are having trouble, please state which steps are confusing you? It would make it easier for us to help.

http://www.mathhelpforum.com/math-he...3f9b2d38-1.gif

Would you accept that this is
$\displaystyle 7 + 1 + \frac{5}{20}$?

What a fraction means is 'top divided by bottom'.

So let's look at the 1.

This could be written as $\displaystyle \frac{2}{2}$ or $\displaystyle \frac{3}{3}$ and so on, as the top divided by the bottom will give one. It could also be written as $\displaystyle \frac{20}{20}$ then, because a fraction means top divided by bottom.

This means we have:
$\displaystyle 7+\frac{20}{20}+\frac{5}{20}$

This means '7, plus five out of twenty, plus twenty of twenty'.

Can you then see that this will equal

$\displaystyle 7 + \frac{25}{20}$?

'seven plus twenty five out of twenty'?