Forms :: Trigger That System Generate Position Against Total Marks?

i have one project about school system.there is one column of total marks.and other one is position column.i want a trigger that system genrate the position against total marks. problem is this when total marks become equal for example 101 and 101 than position also same.but dont now what a trigger i write for this.here some example

if total marks 100 than position is 1stand if total marks 99 than position is 2ndbut third person is also same number in total marks column i mean 99 than in position column must write the 2nd