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

### Instruction:
 哈珀有15个橡皮筋。他的弟弟比他少6个橡皮筋。他们一共有多少个橡皮筋？


### Response:
 他的弟弟有15个橡皮筋 - 6个橡皮筋 = <<15-6=9>>9个橡皮筋。
他们一共有15个橡皮筋 + 9个橡皮筋 = <<15+9=24>>24个橡皮筋。