NDA Math Probability

In month of December 31 daysi.e.(28+3)days.
In 28 days will get 4 Sundays
If we get any Sunday in first 3 days of December than only we can get 5 Sundays in month.
n(5th Sunday)=3   [4 Weeks+3 days]
n(5)=7
Hence prob.of 5 Sundays in month of December =

3

7