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.