1. **Problem statement:** A tank contains 12 litres of oil. The oil is emptied into containers holding 300 ml each. We need to find how many containers are filled. 2. **Important conversion:** 1 litre = 1000 ml. 3. **Convert the total oil volume to millilitres:** $$12 \text{ litres} = 12 \times 1000 = 12000 \text{ ml}$$ 4. **Calculate the number of containers filled:** Each container holds 300 ml, so the number of containers filled is $$\frac{12000}{300} = 40$$ 5. **Answer:** 40 containers are filled. This means the entire 12 litres of oil fills exactly 40 containers of 300 ml each.