\(3\dfrac{1}{3}-2\dfrac{1}{2}\)

\(=\dfrac{10}{3}-\dfrac{5}{2}\)

\(=\dfrac{20}{6}-\dfrac{15}{6}\)

\(=\dfrac{5}{6}\)

\(Ư\left(18\right)=\left\{1;2;3;6;9;18\right\}\)

\(Ư\left(21\right)=\left\{1;3;7;21\right\}\)

\(Ư\left(31\right)=\left\{1;31\right\}\)

\(Ư\left(65\right)=\left\{1;5;13;65\right\}\)

\(\left(\dfrac{5}{84}+\dfrac{5}{204}+\dfrac{5}{374}\right):\left(\dfrac{6}{91}+\dfrac{3}{208}+\dfrac{6}{352}\right)\)

\(=\dfrac{5}{154}:\dfrac{15}{154}\)

\(=\dfrac{1}{3}\)