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? Think carefully first, then make a decision:
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.