$\dfrac{11}{15}:\dfrac{11}{6}+\dfrac{11}{6}:\dfrac{11}{15}$
$=\dfrac{11}{15}.\dfrac{6}{11}+\dfrac{11}{6}.\dfrac{15}{11}$
$=\dfrac{11.6}{15.11} +\dfrac{11.15}{6.11}$
$=\dfrac{66}{165} +\dfrac{165}{66}$
$=\dfrac{66.2}{165.2} +\dfrac{165.5}{66.5}$
$=\dfrac{132}{330} +\dfrac{825}{330}$
$=\dfrac{132+825}{330}$
$=\dfrac{957}{330}$
$=\dfrac{957:33}{330:33}$
$=\dfrac{29}{10}=2,9$