How much heat flows from 20kg of 90 degrees water placed in contact with 20kg of 10 degrees water, producing a final temperature of 50 degrees? c=4186j/kgC
q = mass H2O x specific heat water x delta T. Watch the units.
It doesn't matter which "water" one uses, the change in T is 40 C. Heat flows from the warmer one to the cooler one. .