What is the unknown volume?

Question

An unknown volume of water at 18.2°C is added to 24.3 mL water at 35.6°C. If the final temperature is 23.5°C, what was the unknown volume? (Assume that no heat is lost to the surroundings; d of water is 1.00 g/mL.)

Answer 1

The equation you need is:
q(lost) = -q (gained)
mcDT=-mcDT

where m is the mass of each initial volume of water, c is the specific heat of water (it's going to cancel out) and DT is the final temperature minus the initial temperature of each sample of water.