1. **State the problem:** We need to find how many litres of paint are required to cover a wall that is 3 meters wide and 10 meters long, given that 4 litres of paint can cover 20 square meters. 2. **Calculate the area of the wall:** The area $A$ is calculated by multiplying the width by the length: $$A = 3 \times 10 = 30\text{ m}^2$$ 3. **Determine the paint coverage per litre:** Since 4 litres cover 20 m², 1 litre covers: $$\frac{20}{4} = 5\text{ m}^2 \text{ per litre}$$ 4. **Calculate the litres needed:** To cover 30 m², the number of litres $L$ required is: $$L = \frac{30}{5} = 6$$ 5. **Final answer:** The wall requires **6 litres** of paint to cover it completely.