Use the two steps for solving a linear programming problem, given in the box on page $888,$ to solve the problems.
In 1978 , a ruling by the Civil Aeronautics Board allowed Federal Express to purchase larger aircraft. Federal Express's options included 20 Boeing 727 s that United Airlines was retiring and/or the French-built Dassault Fanjet Falcon $20 .$ To aid in their decision, executives at Federal Express analyzed the following data:
$$\begin{array}{ll}
{} & {\text { Boeing } 727 \quad \text { Falcon } 20} \\
{\text { Direct Operating cost }} & {\$ 1400 \text { per hour } \$ 500 \text { per hour }} \\
{\text { Payload }} & {42,000 \text { pounds } \quad 6000 \text { pounds }}
\end{array}$$
Federal Express was faced with the following constraints:
$\cdot$ Hourly operating cost was limited to 35,000.
$\cdot$ Total payload had to be at least 672,000 pounds.
$\cdot$ Only twenty 727 s were available.
Given the constraints, how many of each kind of aircraft should Federal Express have purchased to maximize the number of aircraft?