Between what levels (in dozens of doughnuts) do the daily sales vary?

Question

A doughnut shop sells a dozen doughnuts for $7.95. Beyond the fixed costs (rent, utilities, and insurance) of $165 per day, it costs $1.45 for enough materials and labor to produce a dozen doughnuts. The daily profit form doughnut sales varies between $400 and $1200. Between what levels (in dozens of doughnuts) do the daily sales vary?

Answer 1

profit=6.6d - 165 =400 when

6.6d = 400+165 = 565 and d = 565/6.6 = about 85.6 dozen donuts

profit = 1200 when

d = 1365/6.6 = about 206.8 dozen donuts

Americans tend to write donuts not doughnuts, as in Dunkin donuts