[Question]: 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]: 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.
So the answer is 280.

[Question]: Maria has a pen and a pencil. She bought the pen for half the price of the pencil. The pencil cost her $8. How much did Maria pay for both the pen and the pencil?
[Answer]: The pen is half the price of the pencil, so it's 8 * 0.5 = $4. So both the pen and the pencil cost Maria 8 + 4 = $12.
So the answer is 12.

[Question]: A bottle can hold 2 cups of water. How many cups of water are needed to fill up 10 whole bottles and 5 half-capacity bottles?
[Answer]:
For 10 whole bottles, you will need 10 * 2 = 20 cups of water. With 5 half - capacity bottles, it requires 5 * 1 = 5 cups of water. In total, you need to have 25 + 5 = 30 cups of water to fill them all.
So the answer is 30.