Temperature Equation: Please!?!

I am so stuck on this problem, my brainiac sister cant even solve it... Please help if your a Math Wizz.. lol Thank u so much !

Given: T = R + (T0 – R) **e **kt , where T is temperature,

T0 is initial temperature, R is room

temperature, t is time in minutes, and k is

a constant you have to calculate.

If the room is 35° F and a boiling cup of water

is 212° F at t = 0 (9:00 AM) and after

t = 20 minutes the temperature is 160° F.

Calculate k for the cup and use the above equation

to predict the temperature at 11:00 AM. This method is used in forensics.