The ratio of books to pens that Arlo has to buy is 7:3.
The total ratio is 7+3 = 10 parts.
If Arlo bought a total of 400 stationery, each part is 400/10 = 40.
So, Arlo bought 7 parts x 40 = <<7*40=280>>280 books.
#### 280
The answer is: 280
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
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.

### Answer:
