Generally for bathing purpose, we heat water to higher temperature say T1 than the temperature at which body is comfortable say T0 (is T0 = 70 degree Celsius?) & add some quantity of cold water to it to have the temperature of water = T0. Is the process energy efficient? What is the best combination of hot & cold water (quantities & temperature) so that least amount of energy is spent? eg, say we require 10 litres of water at temp T0 for comfortable bathing. So how do we do it, heat 7 litres of water to temp T1 & add 3 litres of cold water at temp T2 to get 10 litres of water at temp T0. But this may not be the energy efficient combination, we might heat 10 litres of water to T0 & spend lesser energy than the above case. So what are the equations & how to know which would be the best combination???