WORKDAYS formula in Power Query

Take a look at the M code below. Put it in a blank query. It returns this table:

1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ31DcyMDJQ0lECMSDsWJ1oJROEhIm+kTFUIhYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [#"Start Date" = _t, #"End Date" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Start Date", type date}, {"End Date", type date}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1),
    #"Added Date Range" = Table.AddColumn(#"Added Index", "Date Range", each {Number.From([Start Date])..Number.From([End Date])}),
    #"Expanded Date Range" = Table.ExpandListColumn(#"Added Date Range", "Date Range"),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Date Range",{{"Date Range", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type1", "Is workday", each if Date.DayOfWeek([Date Range],Day.Monday) <= 4 then 1 else 0),
    #"Grouped Rows" = Table.Group(#"Added Custom", {"Start Date", "End Date", "Index"}, {{"Workdays", each List.Sum([Is workday]), type number}}),
    #"Removed Other Columns" = Table.SelectColumns(#"Grouped Rows",{"Start Date", "End Date", "Workdays"})
in
    #"Removed Other Columns"

You should be able to modify it to suite your specific scenario. Ping back if you cannot or need help.