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2015-03-24T03:47:14+05:30

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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.
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  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.
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  The difference between the two numbers :  3/5th of the total number of students.

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