Giải bài 3 trang 178 - SGK Toán lớp 4

a) $\dfrac{2}{5}+ \dfrac{1}{2}+ \dfrac{7}{10}=\dfrac{4}{10}+ \dfrac{5}{10}+ \dfrac{7}{10}= \dfrac{16}{10}=\dfrac{8}{5}$             

b)   $\dfrac{4}{9}+ \dfrac{11}{8}-\dfrac{5}{6}=\dfrac{32}{72}+ \dfrac{99}{72}-\dfrac{60}{72}= \dfrac{32 +99-60}{72}=\dfrac{71}{72}$

c) $\dfrac{9}{20}-\dfrac{8}{15}\times\dfrac{5}{12}= \dfrac{9}{20}-\dfrac{8}{36}= \dfrac{81}{180}-\dfrac{40}{180}=\dfrac{41}{180}$     

d)  $\dfrac{2}{3}:\dfrac{4}{5}:\dfrac{7}{12}=\dfrac{2}{3}\times\dfrac{5}{4}\times\dfrac{12}{7}=\dfrac{5}{6}\times\dfrac{12}{7}=\dfrac{10}{7}$