4 days is 4/7 of a week
Three days is 3/7 of a week, which is a bit less than half.
7/7
It is 4/7 of a week
A fraction requires two numbers. 21 days as a fraction of what? A month, a year, a century?
2/7
Each day is 1/7 of a week (because there are 7 days per week).
4 days is 4/7 of a week
2/7 of a week would be two days since a week is seven days, two 'parts' of a week would be two days.
6/7
It is 5/7, because there are 7 days in a week.
3/7There are 7 days in one week. 3 out of 7 days = 3/7
2/7
Both are acceptable.
Three days is 3/7 of a week, which is a bit less than half.
Since a month consists of four weeks, a fraction of a month would be 1/4th.
It is 2/7.