need computed expression in datatable

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C#

ASP.NET

C#4.0

C#3.5

Hi all,
I had witten below but i am able to calculate i need value in percentage can u guide me or send snippets

DataTable dt=new DataTable();
dt=ds.Tables[0];
DataColumn dc;
dc = new DataColumn("firt",typeof(double));
dc.Expression = "Att_na*100/Att_na+Att_low+Att_medium+Att_high";
dt1.Columns.Add(dc);
dt1.AcceptChanges();

I had wriiten express