I have create a student rank.dbf with five fields namely 1. English 2. Maths 3. Science 4. Total marks 5. Rank.

That is not a good structure for your table. It is not normalizede and not extensible. What happens if they want to add a course called Philosophy? The table should have one record for each course and you can compute the totals and student ranks on the fly for each student using SQL - SELECT.