Below is an instruction that describes a task. Write a response that appropriately completes the request in Chinese. Please answer in Chinese.

### Instruction:
 阿洛购买学习用品的书和钢笔的比例是7:3。如果他从商店购买了总共400个文具，计算他购买的书的数量。


### Response:
 阿洛购买学习用品的总比例是7+3=<<7+3=10>>10
阿洛购买的书的数量是7/10*400=<<7/10*400=280>>280本书。