Giải
Ta có : \(\frac{4}{11}< \frac{x}{20}< \frac{5}{11}\)
\(\Leftrightarrow\frac{80}{220}< \frac{11x}{220}< \frac{100}{220}\)
\(\Leftrightarrow80< 11x< 100\)
\(\Leftrightarrow11\times7,\left(27\right)< 11x< 9,\left(09\right)\)
\(\Leftrightarrow7,\left(27\right)< x< 9,\left(09\right)\)
\(\Leftrightarrow x\in\left\{8;9\right\}\)