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Say I have a data set in the following format: {a:200, b: 100; c:400}

The sum of a, b and c equals 700. Now, I have another value which is supposed to be the actual total say 500.

How do I adjust this difference of 200 in the initial data set (the values for a, b and c must be proportionately decreased summing 200 - something like a weighted difference)?

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Why wouldn't multiplying $a,b,c$ by $\frac57$ work? –  J. M. Dec 22 '11 at 2:28
    
That's correct! But can you elaborate how multiplying by this factor will adjust everything to sum up to the intended total and how that factor is determined? –  dev_musings Dec 22 '11 at 5:34
    
Your $a,b,c$ total up to 700, and you want to "adjust" things so that the total is 500. That means you need to multiply each component by $\frac{500}{700}=\frac57$... –  J. M. Dec 22 '11 at 5:38
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