<human>:  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.
<bot>: How many books does Arlo have to buy? ** The total ratio representing the stationery that Arlo has to buy for his school supplies is 7+3 = <<7+3=10>>10
How many books did Arlo buy? ** 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 = <<7/10*400=280>>280 books.
#### 280