後(の)Noun
Usage
When followed by a number (+ counter) or a noun, means "the remaining" or "the rest of". Here, の after 後 is optional. This use of 後 should be distinguished from 後 in the following example, where 後 means "-more". In this use, の cannot occur after 後.
Examples
このクラスには学生が十五人いる。大学生が十二人、大学院生が一人、後（の）二人は高校生だ。
There are fifteen students in this class: twelve are undergraduates, one is a graduate student, and the remaining two are high school students.
一週間の旅行のうち、三日間は東京に、後（の）四日間は京都に行く予定だ。
During the one-week trip, we plan to spend three days in Tokyo and the remaining four days in Kyoto.