A cup of coffee cools from 90^ C to 80^ C in t minutes when the room temperature is 20^ C . The time taken by the similar cup of coffee to cool from 80^ C to 60^ C at the same room temperature is :

Step-by-step solution

By using average form of Newton's law of cooling aligned & 90-80 t = k ( 90+80 2 -20 ) & 80-60 t ^ = k ( 80+60 2 -20 ) aligned aligned & (i)/(ii) & 10 t ^ t 20 = 65 50 & t ^ = 65 50 2 t = 65 25 t = 13 5 t aligned