Assignment:
A donut shop sells two kinds of donuts. Gourmet donuts sell for $4.15/dozen and regular donuts sell for $3.30/dozen. How many dozen of gourmet donuts must be mixed with 6 dozen of regular donuts to get a mixture that cost $3.90/dozen. Round the answer to the tenths place since I couldn't buy a partial dozen. Please explain.
Provide complete and step by step solution for the question and show calculations and use formulas.