system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If I mix 20 ml of 40 degree Celsius water and we 20 ml of 60 degree water, do I get 40 ml of 50 degree water?

I don't know why, but I can't help but think the answer is a little more complicated than I initially thought. 

EDIT: Thanks for all the answers, smart people. Now I feel ready to finally prepare a cup of green tea correctly. Cheers!
gpt
If you're mixing two of the same substance in the same phase, then yes, it is that simple. If you're mixing different things, then it gets more complicated, and you have to take things like specific heat capacity into account.