The ANSWER!!!!

Guys, question 2 for the pre-lab questions is now available. Here:

STEP 1:
X+Y=150g
 Obviously, X=50, Y=100

This is the total amount of water you have available. You are mixing the water together.


------------------------------------
STEP 2:
X(20)+Y(70)=150(Z)

This is like the concentration and solution problems. You have a 20 celsius water and a 70 celsius water, so they are like different "concentrations" for the different amounts of water.
so just plug in the values and solve for Z.

-------------------------------------
STEP 3:
50(20)+100(70)=150Z
Z=53.33 Celsius.

Hopefully, you understand it. This is just a system of equations.