So apparently it's a "simple" weighted average to calculate the resulting temperature when mixing two volumes of liquid, and the formula doesn't care whether you're using metric or imperial or scientific units (as long as you're consistent).

1 US cup of 50F water + 1 US cup of 200F water = 2 US cups of 125F water

1 dL of 10C water + 1 dL of 100C water = 2 dL of 55C water

3 parts 373K water + 1 part 273K water = 4 parts 348K water
(That's K as in Kelvin.)

(Well, minus whatever you'll lose to the surrounding atmosphere.)

Handy thing to keep in mind if you're trying to brew a hot beverage at a certain temperature, but you don't have one of those newfangled temp-adjustable kettles.

#PocketsLearned