1. **State the problem:** We need to find how many litres of paint are required to paint a wall that is 3 meters wide and 10 meters long if 4 litres cover 20 square meters. 2. **Calculate the area of the wall:** The area $A$ is given by width $\times$ length. $$A = 3 \times 10 = 30 \text{ m}^2$$ 3. **Find litres per square meter:** From the problem, 4 litres cover 20 m², so litres per square meter is $$\frac{4}{20} = 0.2 \text{ litres/m}^2$$ 4. **Calculate number of litres for 30 m²:** Multiply area by litres per square meter: $$30 \times 0.2 = 6 \text{ litres}$$ **Final answer:** $6$ litres of paint are required to cover the wall.