15 Four buoys on the surface of a large, calm lake are located at \(A , B , C\) and \(D\) with position vectors given by
$$\overrightarrow { O A } = \left[ \begin{array} { l }
410
710
\end{array} \right] , \overrightarrow { O B } = \left[ \begin{array} { r }
- 210
530
\end{array} \right] , \overrightarrow { O C } = \left[ \begin{array} { l }
- 340
- 310
\end{array} \right] \text { and } \overrightarrow { O D } = \left[ \begin{array} { c }
590
- 40
\end{array} \right]$$
All values are in metres.
15
- Prove that the quadrilateral \(A B C D\) is a trapezium but not a parallelogram.
15 - A speed boat travels directly from \(B\) to \(C\) at a constant speed in 50 seconds. Find the speed of the boat between \(B\) and \(C\).