Three-quarters of the oil from a 4000-liter tank (that was initially full) was poured into a 20000-liter capacity tanker that already had 3000 liters of oil. How many more liters of oil would be needed to make the large tanker half-full? Let's be accurate as possible.
Three - quarters of oil from the initially full 4000 - liter tank would be 4000 * (3 / 4) = 3000 liters. In addition to the 3000 liters that were already in the large tanker, there are 3000 + 3000 = 6000 liters in it now. The tanker would be filled to half capacity by (1 / 2) * 20000 = 10000 liters of oil. There are 10000 - 6000 = 4000 liters left to reach half capacity.
The answer: 4000.