You don't know where to start ? Start of by calculating the parallel resistance between 20ohm and 30ohm resistor, then maybe something will click off in your mind.
wait .. wait.. whoa whoa whoa.. I am new.. What is this ? Award ? sure, try me .. what you got to do ? Dance and record with your pants down ? I got this, m8.
We care what's at the center of the earth, if you say you take center as RF then your observer is at the centre, and every calculation you do from there on is wrt that observer !