Excel

With this as a table, the following function call can be made from Excel

=IncomeTax(50000.0,TaxRates)

To calculate the tax paid. TaxRates is the range above.

The code for this is as follows.

Function IncomeTax(income As Double, rates As Range) As Double
    Dim i As Integer
    Dim tax As Double
    tax = 0
    For i = 1 To rates.Rows.Count - 1
        tax = tax + Application.Min(income, rates.Cells(i, 1)) * rates.Cells(i, 2)
        income = Application.Max(0, income - rates.Cells(i, 1))
    Next i
    IncomeTax = tax + income * rates.Cells(rates.Rows.Count, 2)
End Function