Formula to deduct from one balance first then the other please?

So i have 100 in A1, and 110 in A2 - we can say they are £ and A1 is winnings A2 is total balance.

(A1 always forms part or whole of A2)

i would like purchases to get deducted from the balance in A2 first. So in the event of a purchase of £5, then just A2 is reduced to 105 - no problem i have that covered.

In the event of a following £25 purchase, (working now from 105), i would like A2 to reduce to 80, but A1 also to reduce to 80...

i also have deposits1 and deposits 2 to cover, but i am happy to come to that, in the hope this is enough information to get started? Purchases can come from B:B if needed.

Any help much appreciated

Thanks Siva, sorry how do i attach an excel?