I have mentioned my problem below  
st = "UPDATE tbl_time set time_out='" + dt1.ToString("hh:mm:ss") + "' WHERE ID=(SELECT MAX(ID) FROM tbl_time)";
 db.ExeQuery(st);
 st = "Select * from tbl_time where ID=(Select MAX(ID) from tbl_time)";
 dr = db.readall(st);
 if (dr.Read() == true)
 {
 txt1.Text = dr["time_in"].ToString();
 txt2.Text = dr["time_out"].ToString();
 }
 TimeSpan t1 = TimeSpan.Parse(txt1.Text);
 TimeSpan t2 = TimeSpan.Parse(txt2.Text);
 TimeSpan ts = t2 - t1;
 dr.Close();
 st = "UPDATE tbl_time set tot='" + ts + "' WHERE ID=(SELECT MAX(ID) FROM tbl_time)";
 db.ExeQuery(st);
 st = "Select * from tbl_time where ID=(Select MAX(ID) from tbl_time)";
 dr = db.readall(st);
 if (dr.Read() == true)
 {
 txt3.Text = dr["tot"].ToString();
 }
 dr.Close();
 TimeSpan t3 = TimeSpan.Parse(txt3.Text);
 double fee = 0;
  
 // In ts variable i'm storing the start and end time i.e total time
 if (ts.TotalMinutes < 60)   // Here is my doubt If the user spend time less than 60 min i want make charge of  10Rs i.e i have done 
 {                                        //but if the user spend 120 min i want make charge of 20Rs, but in the database it saving like 60rs.
   // I'm not getting where my calculation is wrong   
 st = "UPDATE tbl_time set amt='" + fee + "' WHERE ID=(SELECT MAX(ID) FROM tbl_time)";
 db.ExeQuery(st);
 }
 if (ts.TotalMilliseconds > 60)
 {
 fee = (ts.TotalMinutes / 10) * 10;
 st = "UPDATE tbl_time set amt='" + fee + "' WHERE ID=(SELECT MAX(ID) FROM tbl_time)";
 db.ExeQuery(st);
 }
 }
}