SUM（数据）给出不同的输出

Question

我有这个问题：

SELECT finished.first_name, sum(manpower_used.salary) AS 'salary',  
       materials_used.qty * materials_used.price AS 'Total' 
from finished 
INNER JOIN manpower_used ON finished.order_number=manpower_used.order_number
INNER JOIN materials_used ON manpower_used.order_number=materials_used.order_number

我得到了这个输出：

  

first_name |工资|总计为Aaron Sean | 840 | 32.5

我在我的工资栏中得到了错误的输出，正确的输出应该是210（因为我的工资栏中有3个70）但它给了我840

提前谢谢你。