#### On a journey, the average speed of a car is v m s^{-1}. For v ≥ 5, the cost per kilometer, C pence, of the journey is modelled by C = \frac{160}{v} + \frac{v^2}{100}

Using this model,

(a) show, by calculus, that there is a value of v for which C has a stationary value, and find this value of v.