These graph questions are
always difficult given the way the lines spread out at the right side of the
graph.
We get 3400ft from the
graph. Multiply by 1.2 to get 4080ft, close enough to the correct answer. Note
that it says approximately!
The examiner has assured
us that if you go through the graph correctly there will be sufficient spread
between the answers that there will be no doubt as to the correct one!