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A percentile is the value of a variable below which a certain percent of observations fall. So the 20th percentile is the value (or score) below which 20 percent of the observations may be found. The 25th percentile is also known as the first quartile(Q1); the 50th percentile as the median or second quartile(Q2); the 75th percentile as the third quartile (Q3).
For example, assuming there are 100 students admitted to programme X. Admission figure of the 25th percentile means the 25th student admitted according to the merit order list of programme X. If the admitted student number is not equal to 100, you just have to substitute the number of students admitted to a equation:
Let x be the number of students admitted, and y be the actual position from the merit list
y = 25 * (x / 100)