Ariella has $200 more in her son's saving account than Daniella has in her son's savings account. Ariella's account earns her simple interest at the rate of 10% per annum. If Daniella has $400, how much money will Arialla have after two years? Give the rationale and then the answer.
Let's think step by step. If Ariella has $200 more in her son's saving account than Daniella has, then she has $400 + $200 = $600. If she earns an interest of 10% in the first year, her savings account increases by 10 / 100 * $600 = $60. In the second year, she earns the same amount of interest, which $60 + $60 = $120. The total amount of money in Ariella's account after two years is $600 + $120 = $720.. The answer is: 720.

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? Give the rationale and then the answer.
Let's think step by step. 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.. The answer is: 6.

Roselyn gives Mara three times as many books as she gives Rebecca and remains with 60 books. If Rebecca received 40 books, how many books did Roselyn have before? Give the rationale and then the answer.
Let's think step by step. If Roselyn gave Rebecca 40 books, and Mara was given three times as many books, Mara has 3 * 40 = 120 books. Rebecca and Mara's total number of books is 120 + 40 = 160 books. If Roselyn gave Mara and Rebecca her books, which is 160, then she initially had 60 + 160 = 220 books.. The answer is: 220.