Pretty scary if this is considered difficult. I actually had to redo my math's exam in high-school, so I'm trying to be nuanced here, but really, this question should be doable, at least in the Netherlands, when you're 14-15y old, way.. way before the exams. Though I might have a distorted view, since in the Netherlands you are put together with same-leveled candidates at the age of 12.
That type of question always annoyed me, because it asks one thing (time to prey) but you have to solve a different thing (land speed / swimming speed).
Ha! That my friend, is life summed up right there.
If you can't determine that this word problem is a Pythagorean qaudratic, then you might ought not to be sitting a "Higher Maths" exam.
its been a while since I did this math, but isn't it just about taking a derivative and finding the relative minimum/maximum points of the function by where the derivative = 0? (I was on the engineering side of CS, so haven't touched calculus since my undergrad days).