Cumulative calculation based on Ranking

[JOO13]

Dear All, 

I'm trying to create a new column with cumulative calculation based on ranking with below expressions but the output is not as per expected. I have tried multiple expressions as well but doesn't fix. I know this expression should be able to work with just a minor changes, anyone knows what is missing here? 

Dax expression:

Cummulative of Sales by Customers = VAR CurrentRank = 'Sales'[Rank of Sales] RETURN CALCULATE(SUM('Sales'[Sales]), FILTER (ALL('Sales'[Customers]),'Sales'[Rank of Sales]<=CURRENTRANK))

Sample of raw data: (Note: Both "Sales" and "Rank" columns are calculated column)

Sales

Customers	Sales	Rank of Sales
A	20000	1
B	50000	2
C	3000	3

Output from above expression: (The cummulative column basically is just a clone from "Sales" column)

Customers	Sales	Rank of Sales	Cummulative of Sales
A	20000	1	20000
B	50000	2	50000
C	3000	3	3000

Desired output: "Cummulative" column should be sum up the total sales based on ranking 

Customers	Sales	Rank of Sales	Cummulative of Sales
A	20000	1	20000
B	50000	2	70000
C	3000	3	73000

Thank you guys! 

[Zubair_Muhammad · Accepted solution]

@JOO13 

Try

Cummulative of Sales by Customers =
VAR CurrentRank = 'Sales'[Rank of Sales]
RETURN
    CALCULATE (
        SUM ( 'Sales'[Sales] ),
        FILTER ( Sales, 'Sales'[Rank of Sales] <= CURRENTRANK )
    )

[JOO13]

This works like a pro! Thank you, but still couldn't understand why the original expression make a clone from the "Sales" column.