Inverse of a SQL Function

I'm trying to find a way to create a formula to produce the inverse of this function. When writing the answer can you also write down what happens at each step? I tried to break down the formula to see what happens when we do (@StartQtr/10*10) since it looks like a redundant statement but I couldnt see the answer.

GO

/****** Object:  UserDefinedFunction [dbo].[FutureQtr_fn]    Script Date: 08/12/2016 11:52:56 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

-- This function takes a qtr (yyyyq), and adds the number of qtrs, and returns the future qtr
--e.g. FutureQtr (20154,7) returns 20173
CREATE FUNCTION [dbo].[FutureQtr_fn] (@StartQtr int,@AddlQtrs int)

Returns int
AS BEGIN
RETURN
      (@StartQtr/10*10)+ -- Current Year (yyyy)
            ((@StartQtr%(@StartQtr/10*10) +@AddlQtrs - 1)/4)*10 -- add'l years (n0)
            +(@StartQtr%(@StartQtr/10*10) +@AddlQtrs-1)%4  + 1 -- new qtr (n)
END

GO

Select allOpen in new window

Thanks