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

### Instruction:
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.

### Response:
 Ikiwa uwiano wa vitabu na kalamu ambazo Arlo anapaswa kununua kwa vifaa vya shule ni 7:3, na alinunua jumla ya vitabu na kalamu 400, uwiano wa jumla unaowakilisha vifaa vya shule ambavyo alinunua ni 7+3=10
Kipande kinachowakilisha idadi ya vitabu ambavyo Arlo alinunua ni 7/10, na kwa kuwa alinunua jumla ya vitabu na kalamu 400, alinunua 7/10*400=<<7/10*400=280>>280 vitabu.