Hi Scanwinder,
The question you describe is referring to an NDB/ADF problem and not a VOR so that might be part of the confusion for you. COR is an abbreviation meaning "correction" and it is saying we correct the heading by "-30 FROM" to get the bearing FROM the station.
The RBI is simply another word for a fixed card ADF and the indication on the RBI is telling us the relative bearing of the station when compared to the heading of the aircraft.
For example, if the RBI indicates 050 degrees then it is saying the station is 50 degrees to the right of the nose. If we were heading north, then the station must be 50 degrees off to the right of where we are heading. If we were heading 300 degrees and the ADF was indicating 050 then the station must be somewhere 50 degrees off the nose. The magnetic track to the station will be 350 degrees.
The formula for working this out is:
MAGNETIC BEARING = MAGNETIC HEADING + ADF RELATIVE BEARING
if the answer is greater than 360 then simply subtract 360 from the result to get the bearing to the station.
It is possible to make interpreting the RBI a lot easier though. To do this, simply split the face of the instrument into upper and lower halves. All bearing indications in the upper half are giving us bearings TO the station because if we continue, we will fly towards the station. All bearing indications in the lower half are giving us bearings FROM the station because if we continue we will fly further away from the station.
Since the RBI is giving us relative bearings we can simplify things further by considering the indications as being deflections plus or minus a certain number of degrees off the nose or off the tail of the aircraft. For example, if the needle is pointing at 030, it is pointing 30 degrees right of the nose of the aircraft. If the needle is pointing right of the nose we say the deflection is plus, so instead of saying 030 we can say "+30 TO"
Consider an indication of 345 degrees. Now the needle is pointing 15 degrees left off the nose. If the needle is pointing left of the nose we say the deflection is minus, so in this case you would say the the RBI is reading "-15 TO".
Similarly, if the needle is pointing behind us we say the bearing is FROM the station but now we measure the deflection in degrees from the tail of the aeroplane which is represented by the 180 degrees position on the RBI. If the needle is pointing behind us and out to the right (091 degrees through to 179 degrees on the RBI) then we call it minus. If the needle is pointing behind us and to the left (181 degrees through to 269 degrees) we call it plus.
So an indication of 160 degrees would be called "-20 FROM" but an indication of 200 degrees would be called "+20 FROM".
The reason we do this is because it makes it much easier to work out the magnetic bearing to the station without having to worry about subtracting 360 or adding big numbers together. Being simple creatures, we pilots like tricks that keep things simple.
Getting back to your question now, the RBI was reading 150R ( or 150 degrees Relative). In other words the RBI is saying we have a relative bearing of 150 degrees. 150 degrees is in the lower half of the face so we are flying away FROM the station and 150 is 30 degrees to the right of 180 so we can express 150R as "-30 FROM".
To find our magnetic bearing from the station, we combine the heading and the relative bearing as follows:
H +/- A = T
where H = Heading, A = ADF relative bearing (as a plus/minus TO/FROM) and T = magnetic Track.
Getting back to your example, H= 310, A = - 30 FROM and so the magnetic track T from the station is 310 - 30 = 280 FROM
Bob calls this the HAT trick and it is discussed in detail at CPL level.
Interpreting the ADF can be tricky but have another look at the section in the PPL textbook and don't worry too much about it at this stage. At PPL level, the kind of questions you get are very straightforward.
Cheers,
Rich
PS: For more abbreviations, see CASA's list here:
www.casa.gov.au/scripts/nc.dll?WCMS:PWA:604709054:pc=PC_93071 
