Solution
: Let the numbers are $x$ and $y$.
$$
\begin{array}{c}
\therefore \quad x+y=11 \\
x y=18
\end{array}
$$
Dividing eqn. 1 by 2 ,
$$
\begin{aligned}
& \frac{x+y}{x y}=\frac{11}{18} \\
\therefore \quad & \frac{1}{y}+\frac{1}{x}=\frac{11}{18} ; \text { Ans. }
\end{aligned}
$$