The temperature of a body falls from 40 ° C to 36 ° C in 4minutes ,...

Let $\theta$ be instantaneous temperature, $\theta$

For $40 ° C \to 36 ° C$
$\dfrac{36 - 40}{4} = - k ( 38 - 20 ) . . . ( i )$

${θ}_{avg} = 38 ° C$

For $36 ° C \to 32 ° C$
$\dfrac{32 - 36}{t} = - k ( 34 - 20 ) . . . ( ii )$

${θ}_{avg} = 34 ° C$

$\dfrac{( i )}{( ii )}$ Gives

$t = 4\left(\dfrac{18}{14}\right) ⇒ t = 5 . 1min$