一.interest当利息用时 ,为不可数 如：
I borrowed the money at 5% interest.
二.interest当兴趣用时既有可数名词又是不可数名词,
看具体表示什么意思,例如下面的几种用法：
1. 兴趣;关注;爱好
I developed an interest in the machine.
我对这机器发生了兴趣.
I have lost my interest in chemistry.
我对化学已不感兴趣.
2. 趣味性(+to)
Local color added interest to the novel.
地方色彩增加了这部小说的吸引力.
3. 感兴趣的事物或人（这是就可数）
His two great interests in life are music and painting.
三.股份,股权(+in)]
He has an interest in the company.
他在这家公司拥有股份
四. 利润,福利,利益,好处
He always puts the interests of the people before his own.
他总是把人民的利益置于个人利益之上
希望能帮到你,祝更上一层楼O(∩_∩)O