Bonsoir !
[tex]\dfrac{5}{4} \div \dfrac{3}{5}[/tex]
→ [tex]\dfrac{5}{4} \times \dfrac{5}{3}=\boxed{\dfrac{25}{12}}[/tex]
[tex]\dfrac{28}{10} \div \dfrac{21}{15}[/tex]
→ [tex]\dfrac{28}{10} \times \dfrac{15}{21} = \boxed{\dfrac{420}{210}=\dfrac{420 \div 70}{210 \div 70}=\dfrac{6}{3}=\dfrac{6 \div 3}{3 \div 3}=\dfrac{2}{1}=2}[/tex]
Bonne soirée ! ;-)
