let's say the temperature at time t is C(t).at t = 0, C(t) = 95the rate of cooling dC(t)/dt is proportional to the difference in temperature between the coffee and the room.so,
dC(t)/dt is proportional to C(t)-20at
C(t) = 67
dC(t)/dt = 1 = k*67-20
or
k = 1/47
