So, surely, I should keep them? The answer book simply has

I had thought that maybe the answer book was giving a simplified answer, it's an old A-level book, but the syllabus was very different back then, so I'm never sure what I'm expected to know

But looking at it, if I draw a graph, I don't think

*T* can ever - on this graph - be above

*N* on the

*y*-axis, so - presumably - the distance can always be given by

*N* -

*T*, with no need to worry about what would happen if

*T* were to occur above

*N*, giving a negative distance?