Gọi số cần tìm là $x^{}$ ta có :
$\dfrac{11}{18}$ < $\dfrac{19}{x^{}}$ < $\dfrac{11}{17}$
$\Rightarrow$ $\dfrac{209}{342}$ < $\dfrac{209}{11 × x^{}}$ < $\dfrac{209}{323}$
$\Rightarrow$ $342^{}$ > $11 × x^{}$ > $323^{}$
$\Rightarrow$ $11 × x^{}$ $\in$ $B_{(11)}$={ $0^{}$; $11^{}$ ; ... ; $330^{}$ ; $341^{}$ ; ... }
$\Rightarrow$ $11 × x^{}$ = $330^{}$
$11 × x^{}$ = $341^{}$
$\Rightarrow$ $x^{}$ = $30^{}$
$x^{}$ = $31^{}$
