# Calculating a the % of a number to give you a desired result

• Dec 21st 2009, 10:38 AM
Aloupha
Calculating a the % of a number to give you a desired result
Hello,

I am trying to come up with a formula (or use an existing one) to calculate a certain percentage (known) of an unknown number to give a known result.

For example, a problem might come in this form: 2.9% of this number is equal to 675. What is the number?

Any ideas?

Edit: Sorry, I just noticed I posted this in the wrong forum. (I misread Calculator for Calculation). Please delete!
• Dec 21st 2009, 12:32 PM
pickslides
Hi there Aloupha

Quote:

Originally Posted by Aloupha
For example, a problem might come in this form: 2.9% of this number is equal to 675. What is the number?

can be written as

$\displaystyle 2.9\% \times x = 675$

as percent really means "out of 100" then

$\displaystyle \frac{2.9}{100} \times x = 675$

now solving for x you multiply both sides by 100 and divide both sides by 2.9

$\displaystyle x = \frac{675\times 100}{2.9}$

does this make sense?
• Dec 22nd 2009, 06:41 AM
Aloupha
Thank you for your effort pickslides. While what you did did not solve what I seek, it helps me realize what I have been doing wrong. I have done what you suggested before and could not understand why I was not getting the result I was supposed to until I saw what you did (which I did - funny!).

I use paypal for online payment of a small business that I run. PayPal takes a 2.9% plus $0.30 fee off of every payment I receive. For the purpose of figuring out the formula, I decided to first work with the 2.9%. So I need to create a formula to calculate that 2.9% and add it as a processing fee to the client. That way, I will still get the target payment. So my question really is, what number I can take 2.9% off of it that will equal to 675.$\displaystyle x-(\frac{2.9}{100}\times x)= 675$So my formula would be:$\displaystyle x-(\frac{2.9}{100}\times x)+0.30= y$My math is rusty, is this accurate? • Dec 22nd 2009, 12:19 PM pickslides I totally understand the context of the proplem. My Girlfriend is a powerseller on EBay and uses Palpay in majority of her transactions. Given this I am still a little unsure on what you are exactly trying to achieve. Let me guess. You have a selling price in mind$\displaystyle x$in dollars. You want to pass on the$\displaystyle 2.9\%$paypal fee to the seller therefore you need to inflate your selling price? I.e new selling price is$\displaystyle x+\frac{2.9x}{100} = 1.029\times x$• Dec 22nd 2009, 12:33 PM Aloupha I will have some set amounts depending on the package. Say I have 10 packages, each one will have a specific price. Now, I do want to inflate the set price of any given package with the PayPal fee (2.9% + 0.30). The billing invoice will look similar to this. 1. Package ------------------$75.00
2. Processing Fee------------ The Value of 2.9%(of the overall price) + $0.30 Total (overal price)-------- The Value of 1 + 2 ------------------------------------------------------ Working backwards, suppose I had the overall price and it was$75.00. PayPal would take 2.9%+0.30 off of the 75.00 and the remaining will be mine.

It will be like this:

1. Package 1 ---------------- $75.00 2. PayPal Fee --------------$2.47

Balance --------------------- $72.52 Now, I do not want to be getting 72.52, I want my whole 75.00 dollars. So if I can figure out a way to compute the paypal fee when the I know the set price, I can charge my customers the set price + the paypal fee as opposed to just the set price. • Dec 22nd 2009, 12:42 PM pickslides In this case Total$\displaystyle = \$75 + (.029\times \$75) +\$0.3 = \$77.48$• Dec 22nd 2009, 12:56 PM Aloupha Quote: Originally Posted by pickslides In this case Total$\displaystyle = \$75 + (.029\times \$75) +\$0.3 = \$77.48$Exactly! Now, I want to find the formula to calculate this. It will be Y + (0.029 * Y) + 0.3 = X Thanks! Edit: wouldn't work :( • Dec 22nd 2009, 01:00 PM pickslides Ok, so let$\displaystyle x$be the original price and$\displaystyle y$be the inflated price then,$\displaystyle y = 1.029x+0.3$• Dec 22nd 2009, 01:06 PM Aloupha How did you get the 1.029? • Dec 22nd 2009, 01:09 PM pickslides Quote: Originally Posted by Aloupha How did you get the 1.029? from$\displaystyle y = x+0.029x+0.3\displaystyle x+0.029x = 1.029x$therefore$\displaystyle y = 1.029x+0.3$• Dec 22nd 2009, 01:16 PM Aloupha Quote: Originally Posted by pickslides from$\displaystyle y = x+0.029x+0.3\displaystyle x+0.029x = 1.029x$therefore$\displaystyle y = 1.029x+0.3\$

Thanks, that definitely works! I can't believe it was this simple. I was complicating it.

I tested it against some transactions and it gets me very close (rounding issues).