Question

An object kept in a large room having air temperature of 25°C takes 12 minutes to cool from 80° to 70°C. The time taken to cool for the same object from 70°C to 60°C would be nearly :

• A. 10 min
• B. 12 min
• C. 20 min
• D. 15 min

Answer 1

Answer: (D)

15 min

Answer 2

$\frac{T_1 - T_2}{t} = K (\frac{T_1 + T_2}{2} - T_s)$
$\frac{80 - 70}{12} = K( \frac{80 + 70}{2} - 25) \,...(1)$
$\frac{70 - 60}{t} = K(\frac{70 + 60}{2} - 25) \,...(2)$
on solving : $t = 15$ min