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 be accurate as possible.
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.
The answer: 280.