Iam trying to create a simple stock control database , here is my problem:

I have a finished product which consits of 5 sub products in order to make
it therefore if i tell the date base that i have 10 of the 5 sub products in
stock , then i build 1 finished product it should take 1 of each of the sub
products leaving 9 of each sub product left, this is fine , but when i come
to actually sell a finshed product and take it out of stock the finished
product needs to deduct 1 and the sub products should stay the same but i am
unsure of the maths to do this , the way it works now , the sub products
reverts back to 10 can somebody help me with the maths to do this thankyou