Calculate percentage only if all Column is not null using C# in ASP.Net

Below is the structure of my datatable

Id Name Maths Physics Chemistry Score%
1  xyz   10      2               no need
2  abc   8       8        8      24/30*100
3  def   1                       no need   

If we see the datatable i have three records where i need to calculate percentage of score for only one record in my datatable because if a student passes all the subjects only i need to calculate the percentage if he or she fails in any ano of the subject there is no need to display the percentage how can i do this               

I had tried the below code 

                    foreach (DataRow dr1 in dt.Rows)
                    {
                        if (!DBNull.Value.Equals(dr1["Maths"]) && !DBNull.Value.Equals(dr1["Physics"]) && !DBNull.Value.Equals(dr1["Chemistry"]))
                        {
                            double m1 = Convert.ToDouble(dr1["Maths"]);
                            double m2 = Convert.ToDouble(dr1["Physics"]);
                            double m3 = Convert.ToDouble(dr1["Chemistry"]);
                            double totalscore = m1+m2+m3;
                            // Set the three values in the new table now.
                            int maximum = 40;
                            double total = (totalscore / maximum) * 100;
                            dr["Score"] = total;
                        }
                    }

here i am validating for not getting the datatable cell value empty but still even though cell value is empty it is going into the condition and i am getting error how can i resolve this