The way to do it for a multi-guess exam is to choose an answer and then work it out based on that answer. If it doesn't add up, choose another answer in an appropriate direction (either closer or further away depending on how the first answer worked out.)
That might sound like "cheating" or taking the easy way out, but when you're flying with an FMS or GPS you can basically do the same thing except instead of choosing from multi-guess answers, you just choose from enroute waypoints (as described by DirectAnywhere.
BTW, I did ATPL nav a few years back and off track PNRs were part of the exam (I had the joy of trying to work one out with a track that went across two charts, printed on opposite sides of the same piece of paper!)