An airplane flies at a constant altitude of 40,000 feet above sea level. As it starts to land, it descends at a rate of $x$ feet per minute. At what altitude is the plane $y$ minutes after it begins to descend ?

Question

$\begin{array}{1 1} (a)\;40,000 -xy \\ (b)\; 40,000-60xy \\ (c)\; 40,000-x-y \\(d)\; 40,000-60x-y \end{array}$