Tommy ordered a total weight of 10 kilograms of apple, orange, grape, and strawberry fruits. He ordered 3 kilograms of apples, 1 kilogram of orange, and 3 kilograms of grapes. How many kilograms of strawberries did he order? Give the rationale and then the answer.
Let's think step by step. Tommy ordered 3 + 3 + 1 = 7 kilograms of apples, oranges and grapes altogether. Therefore, Tommy ordered 10 - 7 = 3 kilograms of strawberries.. The answer is: 3.

Brian likes to read books. He read one book that had 20 chapters, two books that had 15 chapters each, and one book that had half the chapters of the three previous books put together. How many chapters of books did Brian read? Give the rationale and then the answer.
Let's think step by step. Brian read two books with 15 chapters, so he read 2 * 15 = 30 chapters. He also read a book with 20 chapters, bringing the total for the first three to 30 + 20 = 50 chapters. He then read a book with half the chapters of his cumulative total so far, so it had 50 / 2 = 25 chapters. In total, he read 30 + 20 + 25 = 75 chapters.. The answer is: 75.

Karina was born in 1970 and she is currently twice as old as her brother. If her current age is 40, in what year was her brother born? Give the rationale and then the answer.

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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Let's think step by step. If Karina is currently 40, and she is twice as old as her brother, then her brother is currently 40 / 2 = 20 years old. If Karina was born in 1970, then her brother was born in 1970 + 20 = 1990.. The answer is: 1990.