Give the step-by-step reasoning process and then the final answer. Nate is reading a 400-page book. He finished reading 20% of the book. How many pages does he need to read to finish the book?

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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Nate had read 400 x 20 / 100 = 80 pages. So, he still needs to read 400 - 80 = 320 pages to finish the book.
The final answer: 320.