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Math Help - Calculating a the % of a number to give you a desired result

  1. #1
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    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!
    Last edited by Aloupha; December 21st 2009 at 11:52 AM.
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  2. #2
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    Hi there Aloupha

    Your problem

    Quote Originally Posted by Aloupha View Post
    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

    2.9\% \times x = 675

    as percent really means "out of 100" then

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

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

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

    does this make sense?
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  3. #3
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    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.

    x-(\frac{2.9}{100}\times x)= 675

    So my formula would be:

    x-(\frac{2.9}{100}\times x)+0.30= y

    My math is rusty, is this accurate?
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  4. #4
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    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 x in dollars. You want to pass on the 2.9\% paypal fee to the seller therefore you need to inflate your selling price? I.e new selling price is x+\frac{2.9x}{100} = 1.029\times x
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  5. #5
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    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.
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  6. #6
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    In this case

    Total = \$75 + (.029\times \$75) +\$0.3 = \$77.48
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  7. #7
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    Quote Originally Posted by pickslides View Post
    In this case

    Total = \$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
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  8. #8
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    Ok, so let x be the original price and y be the inflated price then,

    y = 1.029x+0.3
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    How did you get the 1.029?
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  10. #10
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    Quote Originally Posted by Aloupha View Post
    How did you get the 1.029?
    from y = x+0.029x+0.3

    x+0.029x = 1.029x

    therefore y = 1.029x+0.3
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  11. #11
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    Quote Originally Posted by pickslides View Post
    from y = x+0.029x+0.3

    x+0.029x = 1.029x

    therefore 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).
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