a: \(\Rightarrow\dfrac{20}{24}< \dfrac{3y}{24}< \dfrac{30}{24}\)
\(\Leftrightarrow3y\in\left\{21;24;27\right\}\)
hay \(y\in\left\{7;8;9\right\}\)
b: \(\dfrac{2}{13}< \dfrac{1}{y}< \dfrac{3}{4}\)
\(\Leftrightarrow\dfrac{6}{39}< \dfrac{6}{6y}< \dfrac{6}{8}\)
\(\Leftrightarrow6y\in\left\{12;18;24;30;36\right\}\)
hay \(y\in\left\{2;3;4;5;6\right\}\)