อยากทราบการตัดเกรดค่ะ vb ช่วยหน่อยนะคะ ต้องการความช่วยเหลือมากเลยค่ะ!!

Dim dtStudent As New DataTable("Student")
Using cn As New OleDbConnection("Excel Connection String")
    Dim strSQL As String = "Select field1 As stdID, field2 As Total, * From aaa.xls"
    Using da As OleDbDataAdapter = New OleDbDataAdapter(strSQL, cn)
        da.Fill(dtStudent)
    End Using
End Using

'Master Data
Dim dtScore As New DataTable("GradeTable")
dtScore.Columns.Add("FromScore", GetType(Decimal))
dtScore.Columns.Add("ToScore", GetType(Decimal))
dtScore.Columns.Add("Grade", GetType(String))

dtScore.Rows.Add(New Object() {-0.0, 49.99, "F"})
dtScore.Rows.Add(New Object() {50.0, 59.99, "D"})
dtScore.Rows.Add(New Object() {60.0, 69.99, "C"})
dtScore.Rows.Add(New Object() {70.0, 79.99, "B"})
dtScore.Rows.Add(New Object() {80.0, 100.0, "A"})

'Transaction Data
Dim dtStudent As New DataTable("Student")

dtStudent.Columns.Add("stdID", GetType(Integer))
dtStudent.Columns.Add("Total", GetType(Decimal)) ' สอบระหว่างภาค + คะแนนจิตสวาท + etc

dtStudent.Rows.Add(New Object() {3, 95})
dtStudent.Rows.Add(New Object() {1, 50})
dtStudent.Rows.Add(New Object() {2, 66})
dtStudent.Rows.Add(New Object() {4, 75})
dtStudent.Rows.Add(New Object() {5, 49})

Dim g = From x In dtStudent
        From y In dtScore
        Where x.Item("Total") >= y.Item("FromScore") AndAlso x.Item("Total") <= y.Item("ToScore")
        Select StudentID = x.Field(Of Integer)("stdID"), Grade = y.Field(Of String)("Grade")

DataGridView1.DataSource = g.ToList()

'ตารางอัตราภาษีก้าวหน้า 7 ขั้น เริ่มใช้ปี พ.ศ. 2557
Dim dtTaxTable7 As New DataTable("dtTaxTable7")
dtTaxTable7.Columns.Add("IncomeBase", GetType(Decimal))
dtTaxTable7.Columns.Add("IncomeTop", GetType(Decimal))
dtTaxTable7.Columns.Add("TaxRate", GetType(Decimal))
dtTaxTable7.Columns.Add("PrevTop", GetType(Decimal))
dtTaxTable7.Columns.Add("CumulTax", GetType(Decimal))

dtTaxTable7.Rows.Add(New Object() {0.0, 150000, 0, 0, 0})
dtTaxTable7.Rows.Add(New Object() {150000.01, 300000, 0.05, 150000, 0})
dtTaxTable7.Rows.Add(New Object() {300000.01, 500000, 0.1, 300000, 7500})
dtTaxTable7.Rows.Add(New Object() {500000.01, 750000, 0.15, 500000, 27500})
dtTaxTable7.Rows.Add(New Object() {750000.01, 1000000, 0.2, 750000, 65000})
dtTaxTable7.Rows.Add(New Object() {1000000.01, 2000000, 0.25, 1000000, 115000})
dtTaxTable7.Rows.Add(New Object() {2000000.01, 4000000, 0.3, 2000000, 365000})
dtTaxTable7.Rows.Add(New Object() {4000000.01, 1000000000, 0.35, 4000000, 965000})

'ตาราง รายได้สุทธิของพนักงาน พอสังเขป
Dim dtSalary As New DataTable("dtSalary")
dtSalary.Columns.Add("EmpID", GetType(Integer))
dtSalary.Columns.Add("TotalYearlyIncome", GetType(Decimal))
dtSalary.Rows.Add(New Object() {1, 0.99})
dtSalary.Rows.Add(New Object() {2, 149999})
dtSalary.Rows.Add(New Object() {3, 300000})
dtSalary.Rows.Add(New Object() {4, 300000000})
dtSalary.Rows.Add(New Object() {5, 600000000})

Dim t = From x In dtSalary
        From y In dtTaxTable7
        Let Tax = ((x.Item("TotalYearlyIncome") - y.Item("PrevTop")) * y.Item("TaxRate")) + y.Item("CumulTax")
        Where x.Item("TotalYearlyIncome") >= y.Item("IncomeBase") AndAlso x.Item("TotalYearlyIncome") <= y.Item("IncomeTop")
        Select Tax