Er zou moeten gelden dat:

$
\eqalign{
  & \frac{1}
{5} + \frac{1}
{{16}} = \frac{1}
{x}  \cr
  & \frac{{5 + 16}}
{{5 \cdot 16}} = \frac{1}
{x}  \cr
  & \frac{{21}}
{{80}} = \frac{1}
{x}  \cr
  & x = \frac{{80}}
{{21}} = 3\frac{6}
{7} \cr}
$

Jippie!