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For my coffee I need the water to be 90 degrees Celsuis. Boiling water, however, is 100 degrees. How can I make it 90 degrees?

Do I just measure the temperature of the water from my tap, and if it's 10 degrees and I have boiled 6 dl of 100 degree water then I add x such that

$$10x+100 \cdot 6=(x+6)90$$

Or how should I go about it? In the equation above I assumed linearity.

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closed as off-topic by David Z Dec 23 '14 at 11:51

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  • $\begingroup$ That should work. But if you've got a thermometer, you could cheat. Just boil up some water, then keep adding cold water until the temperature drops to 90C. $\endgroup$ – Simon B Dec 23 '14 at 15:34