###Human: 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?, ### Assistant: 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. 
 So the answer is :  4000