A 0.60 kg copper block at 85 °C is put into 0.40 kg of water at 15 °C…

Step 1. Heat capacities. Copper $231\ \text{J/K}$; water $1674\ \text{J/K}$.

Step 2. Balance. $231\ \text{J/K}(85\ ^\circ\text{C} - T) = 1674\ \text{J/K}(T - 15\ ^\circ\text{C})$, so $T = \dfrac{19\,635\ \text{J} + 25\,116\ \text{J}}{1905\ \text{J/K}} = 23.5\ ^\circ\text{C}$.