How to put formula on datagridview column based on virtual column values

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C#

SQL

i want to calculate percentage and show it in datagridview(unbound) percentage column
but i want to calculate percentage from virtual columns selected from tables in query.
this is the query:-

select distinct  s.S_ID,s.Name ,(select  count(Pre) from Attendance where  a.C_ID=C_ID and a.S_ID=S_ID ) as Total_Classes,(select count(Pre) from Attendance where a.C_ID = C_ID and a.S_ID = S_ID and Pre = 'True') as Attended from Course c left outer join StudentCourse sc on c.C_ID = sc.C_ID left outer join Student s on s.S_ID = sc.S_ID left outer  join Attendance a on a.S_ID = s.S_ID and a.C_ID = c.C_ID where c.C_ID ='" + id + "'"

i am getting percentage but the problem is that whenever i click on any column other then Percentage or Present(both these are unbound columns) percentage columns doesnt show any value.
and when datagridview is loaded it shows blank rows it only gets populated after i press percentage or present column and after checking the checkbox in present column it then shows that record

What I have tried:

foreach (DataGridViewRow row in dataGridView1.Rows)
                       {
                double b = double.Parse(dataGridView1.Rows[row.Index].Cells[5].Value.ToString());
                //MessageBox.Show("b="+b);
                double c = double.Parse(dataGridView1.Rows[row.Index].Cells[4].Value.ToString());
                //MessageBox.Show("c="+c);
                int a =Convert.ToInt32(b/c*100);
                dataGridView1.Rows[row.Index].Cells[0].Value = a;