The ratio of books to pens that Arlo has to buy for his school supplies is 7:3. If he bought a total of 400 stationery from the store, calculate the number of books he bought. Let's think first. Chain of thought:
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The total ratio representing the stationery that Arlo has to buy for his school supplies is 7 + 3 = 10. The fraction representing the number of books that Arlo bought is 7 / 10, and since he bought a total of 400 books and pens, he bought 7 / 10 * 400 = 280 books.
Therefore, the answer is 280.
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