Final answer:
The final temperature of a mixture of two equal masses of water at different temperatures will be the average of the two initial temperatures if no heat is lost to the surroundings; in this case, that would be 60°C.
Step-by-step explanation:
The final temperature of water when mixing two quantities at different temperatures can be found by assuming that heat lost by the warmer water is equal to the heat gained by the cooler water until thermal equilibrium is reached, provided no heat is lost to the surroundings. The larger mass of water will influence the mixture's final temperature, as it has a higher heat capacity and will affect the heat exchange process more significantly.
In the scenario described with two equal masses of water at 30°C and 90°C, the final temperature should be exactly halfway between the two if the masses are equal, meaning the resulting temperature would be 60°C. However, if the masses are unequal, the final temperature will be closer to the temperature of the larger mass.