一个女孩去送报纸要36分钟,同样的路程如果他哥哥帮她一起的话,

一个女孩去送报纸要36分钟,同样的路程如果他哥哥帮她一起的话,
问如果她哥哥自己送的话,要多少分钟?
原题是这样的：
It takes a girl 36 minutes to deliver the newspaper on her route,if her brother helps,it takes them only 20 minutes.How long would it take her brother to deliver the newspaper by himself?