Answer:
âθ/ÎT  â(θ-Te)
((100-65))/10 Â â(82.5-21) --------------------------------------------------------------------------(1)
((100-T))/15 Â â(((100+T))/2-21) ---------------------------------------------------------------------------(2)
(2)/(1)
T = 52.73 C
Explanation:
Decreases of temperature is proportional to excess temperature according to the Newtonâs cooling law
âθ/ÎT  â(θ-T1)
Here we have to use θ for average temperature from 100C to 65 C. Because temperature  is not lineally deceased. There for Â
θ=((100+65))/2=82.5 C  Â
Considering 1 Statement,
((100-65))/10 Â â(82.5-21) --------------------------------------------------------------------------(1)
Considering 2 statement,
Assume that final temperature is T after 15 minutes time,
Then ,
θ=((100+T))/2  Â
((100-T))/15 Â â(((100+T))/2-21) ---------------------------------------------------------------------------(2)
(2)/(1)
T = 52.73 C