What is the final temperature when you mix 40ml of 40degree celcius water and 25ml of 20 degree celcius water?
C*40*(40-T) = C*25*(T-20)
C, the specific heat of water, cancels out.
8(40-T) = 5(T-20)
320 +100 = 13 T
Solve for T (the final equilibrium value)