Cell value multiplication, by criteria.

Hello, I am looking for a formula, which could multiply the QTY colum, with all subquantities and write values to Total QTY.
The value would be taken from row where exists Part Number, and muliplied as in the example. Maybe someone has an idea how to make this function?
Thank you!
prb_01.jpg

thank you, Bo_Ry, for your answer.
i get errors, when something is changed in Number (A) column (which should not have any connection in this function)

prob_22.jpg
Attachment 735362

You are changing the way things were presented at first.
Why change 1 to 3 and leave the others unchanged?

Originally Posted by Pepe Le Mokko

You are changing the way things were presented at first.
Why change 1 to 3 and leave the others unchanged?

Hello,
The first column can be changed any time amd they ahould not be a part of the formula.
I showed arrows in the main post, which cells are the keys.
When B cell has a value(any value) its quantity (D) is multiplied to every D cell that has no B cell value and pasted in total quantity area of each row.

Bo_Ry, I would like to thank you again! for help! And I want to apologize, because I was certainly unclear about my request in this thread.

Can I ask to look into this excel file one last time? I fell like it is almost working, but need that "last push".

I have tried using:
=IF(B2="",LOOKUP(2,1/(NOT(ISBLANK(B$2:B2))),D$2:D2)*D2,"Empty Cell")
and
=IF(B3="",LOOKUP(2,1/(B$2:B3<>""),D$2:D3)*D3,"Empty Cell")

it works until the 1st number and then it just shows empty cells.

prob.last.jpg

and I really thank you for your time and help!