Say you have a class with 15 assignments. Over the course of the semester the teacher randomly choses 5 assignments to grade.
The teacher also wants to keep it a surprise, that means the first assignment to be graded can't be assignment 11. Because then you would know that assignments 12, 13, 14, and 15 will be graded.
If you have already turned in some assignments, what would be the chance of todays assignment being graded? Say assignments 2, 5, and 7 have been graded already. What would the chance be that assignment 8 would be graded?