Theo bài ra ta có:
\(\dfrac{2}{7}A=\dfrac{9}{4}.\dfrac{3}{10}B\\ \Rightarrow\dfrac{2}{7}A=\dfrac{27}{40}B\\ \Rightarrow\dfrac{A}{B}=\dfrac{\dfrac{27}{40}}{\dfrac{2}{7}}=\dfrac{189}{80}\)
\(\dfrac{2}{7}\)A=\(\dfrac{3}{10}\).\(\dfrac{4}{9}\)
⇒\(\dfrac{2}{7}\)A=\(\dfrac{2}{15}\)B
⇒\(\dfrac{A}{\dfrac{2}{15}}=\dfrac{B}{\dfrac{2}{7}}\)
\(\dfrac{A}{B}=\dfrac{2}{\dfrac{15}{\dfrac{2}{7}}}\)
⇒\(\dfrac{A}{B}=\dfrac{7}{15}\)