1. **State the problem:** We have a wall that is 20 meters wide and 15 meters high, painted blue. We need to find: A. The area of the wall painted. B. The total amount of paint needed if half a liter covers 1 square meter. 2. **Calculate the area of the wall:** The area $A$ of a rectangle is given by the formula: $$A = \text{width} \times \text{height}$$ Substitute the given values: $$A = 20 \times 15 = 300$$ So, the area of the wall painted is $300$ square meters. 3. **Calculate the total amount of paint needed:** Given that half a liter (\frac{1}{2} liter) covers 1 square meter, the total paint $P$ needed for $300$ square meters is: $$P = 300 \times \frac{1}{2} = 150$$ So, the builder needs $150$ liters of paint to cover the wall. **Final answers:** - Area painted: $300$ m² - Paint needed: $150$ liters