There are four children, one of them is one year younger than the other, multiply their ages together, the product is 11880, what is the age of the youngest one?
有四位小朋友，他们的年龄一个比一个小一岁，将它们的年龄数相乘，积是11880，年龄最小的一个是多少岁．
<result>9</result>