I have just been analysing my mistakes and realised that I broke one of my cardinal rules, one I always bang on about when prepping pupils for their SATs - always read the question carefully!
I measured the distance between two plots and got 77 and selected 77 from the multiple choice, but hadn't noticed that the answer was in statute miles, not nautical miles. The nautical mile option was way off, so I can see why my eye went straight to the 77. What I should have done was convert the 77 nm into kilometres, which is 142.5!
Now I reckon that question is a bit cheeky and I can almost see the Machiavellian grin on the setter's face, but I reckon whoever he is has taught me a valuable lesson: check your units carefully!