What is the final temperature when 100 g of water at 25#"^o#C is mixed with 75 g of water at 50#"^o#C?
You must remeber than total energy remains constant: the thermic energy that wins the cold water must be equal to the thermic energy that loses the hot one.
The heat exchanging of a system with the outside when a mass
Then, if we mix a mass
Substituting the values we have: