###Human: Stella and Twinkle are filling up a truck with a capacity of 6000 stone blocks at the rate of 250 blocks per hour per person. They work for four hours and are then joined by 6 other people who also work at the same rate. How many hours did filling the truck take?, ### Assistant: Stella and Twinkle filled up the truck at the rate of 250 blocks per hour per person, a total of 2*250 = 500 blocks per hour for both.
After working for four hours, Stella and Twinkle had filled 4*500 = 2000 blocks into the truck.
The number of blocks they had to put into the truck for it to be full is 6000-2000 = 4000
When 6 more people joined Stella and Twinkle, a total of 2+6 = 8 people were filling the truck now.
Working at the rate of 250 blocks per person, the eight people filled the truck with 250*8 = 2000 blocks in one hour.
If there were 4000 blocks that still needed to be put into the truck, the 8 people took 4000/2000 = 2 hours to fill the truck with the blocks.
The total time it took to fill up the tank is 4+2 = 6 hours. 
 So the answer is :  6