Solution WILA.M70 Robert Beezer

If the price of standard mix is set at \$5.292, then the profit function has a zero coefficient on the variable quantity $f$. So, we can set $f$ to be any integer quantity in $\set{825,\,826,\,\ldots,\,960}$. All but the extreme values ($f=825$, $f=960$) will result in production levels where some of every mix is manufactured. No matter what value of $f$ is chosen, the resulting profit will be the same, at \$2,664.60.