1. State the problem: We are given that 1 litre of petrol is consumed for every 20 km traveled. 2. We need to find how many litres of petrol are consumed for 850 km. 3. Set up a proportion based on the given rate: $$\frac{1 \text{ litre}}{20 \text{ km}} = \frac{x \text{ litres}}{850 \text{ km}}$$ 4. Solve for $x$ by cross-multiplying: $$x = \frac{850 \times 1}{20}$$ 5. Simplify: $$x = \frac{850}{20} = 42.5$$ 6. Therefore, the vehicle consumes 42.5 litres of petrol for 850 km.