Stella and Twinkle filled up the truck at the rate of 250 blocks per hour per person, a total of 2*250 = <<250*2=500>>500 blocks per hour for both.
After working for four hours, Stella and Twinkle had filled 4*500 = <<4*500=2000>>2000 blocks into the truck.
The number of blocks they had to put into the truck for it to be full is 6000-2000 = <<6000-2000=4000>>4000
When 6 more people joined Stella and Twinkle, a total of 2+6 = <<2+6=8>>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 = <<250*8=2000>>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 = <<4000/2000=2>>2 hours to fill the truck with the blocks.
The total time it took to fill up the tank is 4+2 = <<4+2=6>>6 hours.
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