[Required] To find three quantities \(x\), \(y\), and \(z\), where the sum of any two is known.

*Universal Arithmetick*, Isaac Newton, 1728

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The answer is: if you label the three sums as \(x+y=a\), \(x+z=b\) and \(y+z=c\), then \(x = \dfrac{a+b-c}{2}\), \(y = \dfrac{a-b+c}{2}\) and \(z=\dfrac{-a+b+c}{2}\).