I'm still having trouble with the question, and I'm having trouble with two more. Could anyone help with any?
11)
'A boat P, capable of 6 m s-1 in still water, travels from a point A around a course ABC which is an equilateral triangle with sides 500 m long. A uniform current of 4 m s-1 flows in the direction AB->. An identical boat Q starts from A at the same time and travels the other way (ACB) around the course. Which boat gets back to A first, and what is the margin of victory (in seconds)?'
12)
'A river is D m wide and flows at u m s-1. A man can swim at v m s-1 in still water, where v > u. Find the ratio of the shortest time it would take him to swim across the river and back, and the time it would take him to swim D m upstream and back.'
14)
'OABC is a tetrahedron. L, M and N are the mid-points of OA, OB and OC respectively. P, Q and R are the mid-points of BC, AC and AB respectively. OA-> = a, OB-> = b and OC-> = c. Use vector methods to show that the lines PL, QM and RN bisect each other.'