Certain quantity of water cools from 70° C to 60° C in the first 5 minutes and to 54°C in the next 5 minutes. The temperature of the surroundings is:-

45°C 20° 42°C 10°C

45°C

\(\frac{T_1 + T_2}{\Delta t} = k[\frac{T_1 + T_2}{2} - T_o]\) i) T1 = 70°C ; T2 = 60°C  ⇒ \(\frac{10}{5} = k[65 - T_o]\) ii) T1 = 60°C ; T2 = 54°C  ⇒ \(\frac{6}{5} = k[57 - T_o]\) To = 45°C