A teacher has 100 students and wants to divide them into groups for a project. Each group must have the same number of students, and there must be more than one group. Additionally, the number of students in each group must be a multiple of 5. If a student is randomly selected from the 100 students, what is the probability that the student belongs to a group with more than 4 but fewer than 21 students? Let's think step by step and output the final answer within \boxed{}.
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