1. The problem is to find the linear equation of the form $C = mV + b$ that fits a straight line on a graph with given points. 2. From the description, the line passes near points $(0, 20)$ and $(40, 100)$. 3. Calculate the slope $m$ using these points: $$m = \frac{100 - 20}{40 - 0} = \frac{80}{40} = 2$$ 4. The y-intercept $b$ is the value of $C$ when $V = 0$, which is approximately 20. 5. Therefore, the linear equation representing the cost is: $$C = 2V + 20$$ 6. This means the cost increases by 2 units for each additional litre ordered, starting from a base cost of 20 when no volume is ordered.