Bài tập 3 trang 116 VBT Toán 4 tập 2

a) \(\dfrac{3}{4} + \dfrac{1}{2} + \dfrac{5}{8} = \dfrac{{6 }}{8} + \dfrac{4}{8}+ \dfrac{5}{8} \)\(= \dfrac{{10}}{8} + \dfrac{5}{8} = \dfrac{{15}}{8}\)

b) \(\dfrac{5}{7} + \dfrac{9}{8} - \dfrac{{15}}{{28}} = \dfrac{{40 }}{{56}}+ \dfrac{63}{56} - \dfrac{{15}}{{28}}\)\(= \dfrac{{103}}{{56}} - \dfrac{{15}}{{28}} = \dfrac{{103 }}{{56}}- \dfrac{30}{56} = \dfrac{{73}}{{56}}\)

c) \( \dfrac{{11}}{{30}} - \dfrac{9}{{25}} \times \dfrac{{15}}{{27}} = \dfrac{{11}}{{30}} - \dfrac{{9 \times 15}}{{25 \times 27}}\)\(=  \dfrac{{11}}{{30}} - \dfrac{{9 \times 5 \times 3}}{{5 \times 5 \times  9\times 3}}= \dfrac{{11}}{{30}} - \dfrac{1}{5}\)\(= \dfrac{{11 }}{{30}}-  \dfrac{6}{30}= \dfrac{5}{{30}} = \dfrac{1}{6}\)

d) \(\dfrac{3}{4}:\dfrac{5}{8}:\dfrac{3}{{10}} = \dfrac{3}{4} \times \dfrac{8}{5} \times \dfrac{{10}}{3} \)\( =\dfrac{3 \times 8 \times 10}{4 \times 5 \times 3}= \dfrac{3 \times 4 \times 2 \times 5 \times 2}{4 \times 5 \times 3}\)\(= 4\)

-- Mod Toán lớp 4 HỌC247