I hope I'm not attempting someone's homework, especially if I get it wrong, but here goes.
a1= (x/12)+y+z so
a1= (x/12)+w
a1= (.15w/12)+w
12*a1=.15w+12w
12*a1=12.15w
w=(12*a1)/12.15
x=.15w
y=1-x
z=.15y
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Thanks guys! So using the following worked well using pencil & paper:
x = .15w/12
y = .85w
z = .85w * .15
a1 = (.15w/12) + .85w + (.85w * .15w)
So - using pencil & paper, I can solve this now BUT I want to solve it in excel where A1 is the value in cell A1 and I want x to be returned in A2, y in A3 and z in A4.