Exponential and Logarithmic Functions · Superchallenge problem

A cup of tea cools according to Newton's law of cooling, $T(t)=R+Ae^{-kt}$, where $R$ is the (constant) room temperature. The room temperature is not known. The tea measures $92^\circ$C at $t=0$, $56^\circ$C at $t=10$ minutes, and $38^\circ$C at $t=20$ minutes. Find $R$ in degrees Celsius.