That will work. Notice that you are using math though. In a brilliant simplification you have taken i=0 and Tx = Tw = 65. That means that,
Tf = 65*x/5+65*w/5 = 65*(x+w)/5 = 65, since x+w =5, the total volume of mixed wort when ice is not used ;).
With your set-up, if (for example) you had...