Giải bài 3 trang 136 - SGK Toán lớp 4

a) $\dfrac{2}{3} \times\dfrac{5}{7}=\dfrac{2\,\times\,5}{3\,\times\,7} =\dfrac{10}{21}$       

$\dfrac{10}{21} :\dfrac{5}{7}= \dfrac{10}{21} \times\dfrac{7}{5}=\dfrac{70}{105}=\dfrac{2}{3}$                     

$\dfrac{10}{21} :\dfrac{2}{3}=\dfrac{10}{21} \times\dfrac{3}{2}=\dfrac{30}{42} =\dfrac{5}{7}$                 

b) $\dfrac{1}{5} \times\dfrac{1}{3}=\dfrac{1}{15}$       

$\dfrac{1}{15} :\dfrac{1}{5}=\dfrac{1}{15} \times\dfrac{5}{1}=\dfrac{5}{15} =\dfrac{1}{3}$     

$\dfrac{1}{15} :\dfrac{1}{3}=\dfrac{1}{15} \times\dfrac{3}{1}=\dfrac{3}{15} =\dfrac{1}{5}$