I'm starting a construction agency. When a contractor bills a customer, I will bill him 10% of his invoice as my commission for finding him the work. The contractor will want to add on my commission, so that after he's paid my bill, he will be left with what he would have originally charged. However, if (for example) he was to charge £1000 for his work, and he added 10% (to cover my commission and not be out of pocket) his bill would be £1100. But when I then charge him my 10% fee (£110), he's left with £990, so is £10 out of pocket. So what formula will he need to use, so that he ends up with his 'planned' invoice total after my commission. Is there a formula that will work for this, regardless of the invoice (i.e. always end up with the original number after the 10% deduction)? Thanks!