The function's difference quotient is needed for this. Differentiating the expression for c(x) we get -40+0.2x. This the gradient of the graph of the function. When the gradient is zero, we have a maximum or minimum. The shape of this graph is a narrow U, so the gradient is zero at the minimum point. Solving -40+0.2x=0, we get x=40/0.2=200. So 200 trucks incur minimum cost.