Hi nauna,

Refer below sample.

HTML

Percentage : <asp:TextBox runat="server" ID="txtPercentage" /><br />
<asp:Button ID="Button1" Text="Convert" runat="server"  OnClick="ConvertData"/>

Code

C#

protected void ConvertData(object sender, EventArgs e)
{
    double percentage = Convert.ToDouble(txtPercentage.Text);
    double convertToDecimal = percentage / 100;
    string fraction = DecimalToFraction(convertToDecimal);
    ScriptManager.RegisterStartupScript(this, this.GetType(), "Message", "alert('" + fraction + "');", true);
}

public string DecimalToFraction(double value)
{
    string result;
    double numerator, realValue = value;
    int num, den, decimals, length;
    num = (int)value;
    value = value - num;
    value = Math.Round(value, 5);
    length = value.ToString().Length;
    decimals = length - 2;
    numerator = value;
    for (int i = 0; i < decimals; i++)
    {
        if (realValue < 1)
        {
            numerator = numerator * 10;
        }
        else
        {
            realValue = realValue * 10;
            numerator = realValue;
        }
    }
    den = length - 2;
    string ten = "1";
    for (int i = 0; i < den; i++)
    {
        ten = ten + "0";
    }
    den = int.Parse(ten);
    num = (int)numerator;
    result = SimplifiedFractions(num, den);
    return result;
}

string SimplifiedFractions(int num, int den)
{
    int remNum, remDen, counter;
    if (num > den)
    {
        counter = den;
    }
    else
    {
        counter = num;
    }
    for (int i = 2; i <= counter; i++)
    {
        remNum = num % i;
        if (remNum == 0)
        {
            remDen = den % i;
            if (remDen == 0)
            {
                num = num / i;
                den = den / i;
                i--;
            }
        }
    }

    return num.ToString() + "/" + den.ToString();
}

VB.Net

Protected Sub ConvertData(ByVal sender As Object, ByVal e As EventArgs)
    Dim percentage As Double = Convert.ToDouble(txtPercentage.Text)
    Dim convertToDecimal As Double = percentage / 100
    Dim fraction As String = DecimalToFraction(convertToDecimal)
    ScriptManager.RegisterStartupScript(Me, Me.GetType(), "Message", "alert('" & fraction & "');", True)
End Sub

Public Function DecimalToFraction(ByVal value As Double) As String
    Dim result As String
    Dim numerator As Double, realValue As Double = value
    Dim num, den, decimals, length As Integer
    num = CInt(value)
    value = value - num
    value = Math.Round(value, 5)
    length = value.ToString().Length
    decimals = length - 2
    numerator = value
    For i As Integer = 0 To decimals - 1
        If realValue < 1 Then
            numerator = numerator * 10
        Else
            realValue = realValue * 10
            numerator = realValue
        End If
    Next
    den = length - 2
    Dim ten As String = "1"
    For i As Integer = 0 To den - 1
        ten = ten & "0"
    Next
    den = Integer.Parse(ten)
    num = CInt(numerator)
    result = SimplifiedFractions(num, den)
    Return result
End Function

Private Function SimplifiedFractions(ByVal num As Integer, ByVal den As Integer) As String
    Dim remNum, remDen, counter As Integer
    If num > den Then
        counter = den
    Else
        counter = num
    End If

    For i As Integer = 2 To counter
        remNum = num Mod i
        If remNum = 0 Then
            remDen = den Mod i
            If remDen = 0 Then
                num = num / i
                den = den / i
                i -= 1
            End If
        End If
    Next
    Return num.ToString() & "/" & den.ToString()
End Function

Screenshot

For more acurate result refer below link.

Convert percentage (Double) to Fraction using C# in ASP.Net