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

### Instruction:
 瑞安比希西家大7岁。瑞安和希西家的年龄加起来等于15岁。希西家多大？


### Response:
 设H为希西家的年龄
瑞安 = H + 7
2H + 7 = 15
2H = 8
H = <<4=4>>4
希西家4岁。