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Math Help - Formula Needed

  1. #1
    Goldfinger
    Guest

    Formula Needed

    Hi All

    I need a formula for a hobby of mine. I use clay to make things. As it dries- it also shrinks. Depending on which type of clay I use- it can shrink anywhere between 10%-28%.

    My question is this (as an example)- assuming I want to wind up with a dried ,finished product with diameter of 25 cms and the clay I'm using shrinks by 12%- What would I need to start off with before I start the drying process? I know most people will multiply 25cms times 12% - then add it to the 25cms for the answer. But 3cms + 25cms= 28cms. 28cms x 12%=3.36cms 28cms minus 3.36cms=24.64cms.
    Close - but not close enough.


    Anyone have a formula that will work more precisely?

    Thanks

    Robert
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  2. #2
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    Quote Originally Posted by Goldfinger View Post
    Hi All

    I need a formula for a hobby of mine. I use clay to make things. As it dries- it also shrinks. Depending on which type of clay I use- it can shrink anywhere between 10%-28%.

    My question is this (as an example)- assuming I want to wind up with a dried ,finished product with diameter of 25 cms and the clay I'm using shrinks by 12%- What would I need to start off with before I start the drying process? I know most people will multiply 25cms times 12% - then add it to the 25cms for the answer. But 3cms + 25cms= 28cms. 28cms x 12%=3.36cms 28cms minus 3.36cms=24.64cms.
    Close - but not close enough.


    Anyone have a formula that will work more precisely?

    Thanks

    Robert
    hmmm.

    25=(1-.12)a

    Therefore: 25=0.88a

    Divide: \frac{25}{0.88}=a

    Finally: 28.4090909=a

    Therefore the formula is: \boxed{\frac{\text{the amount wanted}}{1-\text{the percent loss}}=\text{the amount you should start with}}
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