SI. 1 The sum of two numbers is 40 , their product is 20 . If the sum of their reciprocals is $a$, find $a$. Let the two numbers be $x, y$.
$\begin{aligned} x & +y=40 ; x y=20 \\ a & =\frac{1}{x}+\frac{1}{y} \\ & =\frac{x+y}{x y} \\ & =\frac{40}{20}=2\end{aligned}$