I needed hot water at around 80-85 Celsius to use with AeroPress. At the time, I didn't have a thermometer that goes higher than 50C. I know the room temperature and the temperature of near-boiling water (close to 100C ). The result: a calculator, for a cool (pun intended), practical use of thermodynamics.
How many grams of ~20C water are needed to lower the temperature of 500g of ~100C water to ~85C?
Here is a calculator to do that for us. Go ahead and change the values for desired temperature and room/ambient temperature to get your answer.
(Note: desired temperature should be between Cold and Hot water temperatures)
Via convection, heat transfers from the hotter liquid to the colder.
We can calculate heat transfer quantity using
Putting those two together we get
$$Q1 + Q2 = 0$$
$$M1•C•(T - T1) + M2•C•(T- T2) = 0$$
$$500•C•(85 - 100) + M2•C•(85 - 20) = 0$$
$$500•C•(-15) + M2•C•(65) = 0$$
$$M2•C•(65) = 500•C•(15)$$
And, solving for M2, we get
I need to add 115.3g of room-temperature water.