<TextBlock x:Name="TextDisplay" TextAlignment="Center"
                       Style="{StaticResource PhoneTextTitle1Style}" />


<Button Style="{StaticResource MyButtonStyle}" Content="1" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="2" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="3" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="4" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="5" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="6" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="7" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="8" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="9" Click="Number_Click" />
<Button Style="{StaticResource MyButtonStyle}" Content="0" Click="Number_Click" />

private void Number_Click(object sender, RoutedEventArgs e)
    {
        Button b = (Button)sender;
        TextDisplay.Text += b.Content.ToString("###.##");
    }