• Welcome! The new FreeMathHelp.com forum is live. We've moved from VB4 to Xenforo 2.1 as our underlying software. Hopefully you find the upgrade to be a positive change. Please feel free to reach out as issues arise -- things will be a little different, and minor issues will no doubt crop up.

HELP: Finding the Percent Change

lillybeth

Junior Member
Joined
Nov 1, 2012
Messages
211
Hello everybody!!! I need help with understanding how to find percent change.


My book gives this formula: (amount of change/original amount) * 100%



For a problem that looks like this: 80 to 100, I usually just do 20/80, which is equal to 25% (which my book's answer key tells me I am correct)

Soooo.

For a problem that reads 200 to 100, would you do 100/200, or -100/200??

And one more thing:

The formula our book tells us to use says to multiply the amount of change divided by the original amount by 100% (1.00).

What is the point of multiplying by one?? :(




Thank you guys!!!
All help will be greatly appreciated!!! :) ;)
 
Last edited:

pka

Elite Member
Joined
Jan 29, 2005
Messages
7,685
Hello everybody!!! I need help with understanding how to find percent change.
My book gives this formula: (amount of change/original amount) * 100%

For a problem that looks like this: 80 to 100, I usually just do 20/80, which is equal to 25% (which my book's answer key tells me I am correct)

For a problem that reads 200 to 100, would you do 100/200, or -100/200??
It is \(\displaystyle \dfrac{100-200}{200}=-\dfrac{100}{200}=-50\%\).

It is always \(\displaystyle \dfrac{\text{new price}-\text{old price}}{\text{old price}} \).
 

Ishuda

Elite Member
Joined
Jul 30, 2014
Messages
3,345
Percent means 'amount of parts out of 100 parts'. To use your example the change for 80 to 100 fractional change would be \(\displaystyle \frac{100-80}{80} = 0.25\). So for every one part you had, it would change by 0.25 parts. If you had a hundred parts, it would change by 100*0.25=25 parts which is 25 parts per hundred or 25%.

Thus compute the fractional change, see pka's post, then multiple by 100 to get the percent change. That is what is meant in your equation of "multiply by 100%"
 

lillybeth

Junior Member
Joined
Nov 1, 2012
Messages
211
It is \(\displaystyle \dfrac{100-200}{200}=-\dfrac{100}{200}=-50\%\).

It is always \(\displaystyle \dfrac{\text{new price}-\text{old price}}{\text{old price}} \).

Thanks!!! But what about my second question??
 

Subhotosh Khan

Super Moderator
Staff member
Joined
Jun 18, 2007
Messages
18,086
Thanks!!! But what about my second question??
We multiply by 100% - so that we can have the answer as "%" as opposed to a fraction or a decimal number (without %).
 

lillybeth

Junior Member
Joined
Nov 1, 2012
Messages
211
We multiply by 100% - so that we can have the answer as "%" as opposed to a fraction or a decimal number (without %).
Ok thx!!!!! I was just curious because there seemed no point really to multiply by one.

Thank you soo much!!!

:) :p
 
Top