calculate data in the same cell in excel

calculate data in the same cell in excel

im not sure how to do this, i was hoping someone could help me out here.
i would like a cell to automatically update a mathematical equation after
entering a number. The reason i want to do this is because each
transaction i have costs 10 cents, plus 2% fee for credit cards.
So for example, transactions(D15)=21 Amount(E15)=108
Where the amount is, i would like the cell to update with Transactions
cell in this case
(D15*0.10)-(E15-2%) = 103.78
So E15 will automatically update with 103.78.
The reason why is i have to copy and past and i would like it to update in
the same cell.
Anyone? this would be great. thanks allot. im not sure if this would be
Macro or i can insert it as a cell function??