How to use reverse SUM ?

I'm total newbie in Excel and I need just one formula. It could be tricky one or (I hope) is simple.

SUM is very simple. I would like to reverse it!

Sample - different paint tinters which I'm using to make a formula to make a color.

AM01 white - 120
AM06 black - 80
AM64 magenta -800

SUM - 1000

That's very ease. :-)

I would like to reverse it!


If I want not 1000 but 4000 (4l) or 20000.

I mean - if I will put 4000 in sum cell, all three components - colors with change automatically to:

AM01 white - 480
AM06 black - 320
AM64 magenta -3200

SUM - 4000


I need it for paint mixes and if I want different quantity, (240, 1545, etc) all I can do is to put such amount, and all components will be adjusted automatically. There are some programs for paints (DUPONT, STANDOX) who are making such programs. It's not for them!

Can this be done easy in Excel?



PLEASE HELP!

TIA