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.
The total ratio representing the stationery that Arlo has to buy for his school supplies is 7+3 = 
<gadget id="calculator">7+3</gadget>
<output>10</output>
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 = 
<gadget id="calculator">7/10*400</gadget>
<output>280</output>
280 books.

<result>280</result>