G6.3 The sum of two numbers is 10 , their product is 20 . The sum of their reciprocal is $c$. What is $c$ ?
Let the two numbers be $x, y$.
$$
\begin{array}{l}
x+y=10 \ldots \ldots .(1) \\
x y=20 \ldots \ldots(2) \\
c=\frac{1}{x}+\frac{1}{y}=\frac{x+y}{x y}=\frac{10}{20}=\frac{1}{2}
\end{array}
$$