7. Project: Auditing Someone Else's Model

You've been handed a profit model for a home-appliance store. The author has left, and the numbers "don't add up." Your job is to audit the model and bring it up to a professional standard:

• cost of goods and the marketplace fee are calculated with numbers inside the formulas (=D2*0.6, =D2*0.08), even though the parameters sit in the block J1:J3;
• in one row the cost is typed in as a number;
• the "Total" row doesn't cover the whole range, and the profit total is simply typed in by hand;
• there is no tax and no check cell.

Use the techniques from this track: Ctrl+`, Trace Precedents, references to parameters, and a check total. At the end, test the model by changing a parameter: the marketplace fee rises to 10%.