Given, In case 1 , body cools from temperature 70∘ to 40∘C i.e. T1=70∘C,T2=40∘C and time t1=5min In case 2 , temperature reduces from 60∘C to 40∘C in time (t2) i.e, T3=60∘C,T4=40∘C,t2= ? Consider temperature of surrounding, Ts=20∘C. By using Newton's law of cooling, ‌
Ti−Tf
t
=K(‌
Ti+Tf
2
−Ts). . . (i) where, Ti,Tf be initial and final temperature of body, t is the time taken by body to reach Ti to Tf and K is thermal heat coefficient. ccording to first case, By using Eq. (i), we get ‌‌
T1−T2
t1
‌‌=K(‌
T1+T2
2
−Ts) ∴‌‌
70−40
5
‌‌=K(‌
70+40
2
−20) ⇒‌‌
30
5
‌‌=K(‌
110
2
−20)⇒6=K(35) ⇒‌K‌‌=‌
6
35
According to second case, again by using Eq. (i), we get ‌