The combined weight of a motorbike and rider is $1800\,\text{N}$. The motorbike moves in a straight line at steady speed up a hill that is inclined at $30^\circ$ to the horizontal. The useful output power of the motorbike is $36\,000\,\text{W}$. The overall resistive force from air resistance and friction acting on the motorbike and rider is $2400\,\text{N}$. Calculate the speed of the motorbike.
- A$8.6\,\text{m s}^{-1}$
- B$11\,\text{m s}^{-1}$
- C$15\,\text{m s}^{-1}$
- D$24\,\text{m s}^{-1}$