Multi level Bill of material

hello everybody
can anyone help me to create a multi level BOM in excel:
i have a formula
A=a+b+c+B
B=a+d+e

if i select A, i need excel to give 2a+b+c+d+e (and that should be in another sheet.
also i may take 50% of A +50% of B the resulting formula must appear.
i attached an exemple file.
thanks in advence.