Let there be 100 parts of the test. Let there be N total number of students writing the test. The number of students at each level of completion of 100 parts ie., 0%, 1%, 2%, 3%,..15%,...25%,...50%,..75%,,98%,..,100% are equal. Let this number be n.
Thus the total number of students = n * 100 = N
n = N/100
The number of students who completed 4/5ths ie., 80% of the test will be those who are attempting 81st part, 82st part, ...100th part. of the test.
n * 20 = 20% N => 1/5 th of the total number of students.
Number of students who completed 20% of the test will be those who are currently writing 21st,22nd,..50th..60th..80th...100th part of the test.
Hence they are n * 80 = 80% N or 4/5 th of the total number of students.
The difference between the two numbers : 3/5th of the total number of students.