Initially, mean temperature difference \( = \left( {\frac{{70 + 60}}{2} - {\theta _0}} \right) = (65 - {\theta _0})\) Finally, mean temperature difference \( = \left( {\frac{{60 + 50}}{2} - {\theta _0}} \right) = (55 - {\theta _0})\)
In second case mean temperature difference decreases, so rate of fall of temperature decreases, so it takes more time to cool through the same range.