\(x\times2=\dfrac{5}{9}\)

\(x=\dfrac{5}{9}:2\)

\(x=\dfrac{5}{18}\)

____________

\(x:\dfrac{3}{5}=4\)

\(x=4\times\dfrac{3}{5}\)

\(x=\dfrac{12}{5}\)

__________________

\(15-x=\dfrac{1}{2}\)

\(x=15-\dfrac{1}{2}\)

\(x=\dfrac{29}{2}\)

_______________

\(x+\dfrac{1}{4}=\dfrac{3}{2}\)

\(x=\dfrac{3}{2}-\dfrac{1}{4}\)

\(x=\dfrac{5}{4}\)

\(\left(-2\right).\left(-\dfrac{-7}{12}\right)=\dfrac{14}{12}=\dfrac{7}{6}\)

\(16,\dfrac{-7}{3}+\dfrac{-5}{6}=\dfrac{-14}{6}+\dfrac{-5}{6}=\dfrac{\left(-14\right)+\left(-5\right)}{6}=\dfrac{-19}{6}\)

\(17,\dfrac{7}{2}-\dfrac{9}{4}=\dfrac{14}{4}-\dfrac{9}{4}=\dfrac{14-9}{4}=\dfrac{5}{4}\)

\(19,\dfrac{15}{4}-\dfrac{7}{2}=\dfrac{15}{4}-\dfrac{14}{4}=\dfrac{1}{4}\)

\(20,\dfrac{5}{3}-\dfrac{4}{-21}=\dfrac{35}{21}-\dfrac{-4}{21}=\dfrac{35-\left(-4\right)}{21}=\dfrac{39}{21}=\dfrac{13}{7}\)

\(22,\dfrac{8}{15}+\dfrac{-7}{20}=\dfrac{32}{60}+\dfrac{-21}{60}=\dfrac{32+-21}{60}=\dfrac{11}{60}\)

\(23,\dfrac{5}{8}-\dfrac{7}{10}=\dfrac{25}{40}-\dfrac{28}{40}=\dfrac{25-28}{40}=\dfrac{-3}{40}\)

\(25,\dfrac{-1}{9}+\dfrac{5}{18}=\dfrac{-2}{18}+\dfrac{5}{18}=\dfrac{-2+5}{18}=\dfrac{3}{18}=\dfrac{1}{6}\)

\(26,\dfrac{3}{4}-\dfrac{5}{12}=\dfrac{9}{12}-\dfrac{5}{12}=\dfrac{9-5}{12}=\dfrac{4}{12}=\dfrac{1}{3}\)

\(28,\dfrac{-4}{7}+\dfrac{-5}{10}=\dfrac{-40}{70}+\dfrac{-35}{70}=\dfrac{-75}{70}=\dfrac{-15}{14}\)

\(29,\dfrac{-7}{15}-\dfrac{3}{25}=\dfrac{-35}{75}-\dfrac{9}{75}=\dfrac{-35-9}{75}=\dfrac{-44}{75}\)

\(5=5\)

\(8=2^3\)

\(12=2^2.3\)

\(BCNN\left(5;8;12\right)=2^3.3.5=120\)

Theo đề bài : Số học sinh khối 6 của 1 trường khoảng gần 500 học sinh : \(B\left(120\right)=\left\{0;120;240;360;480;600;...\right\}\) Vậy 480 học sinh thỏa mãn yêu cầu đề bài, nhưng nếu xếp 5,8,12 hàng sẽ thiếu 1 học sinh, nên \(480-1=479\left(hs\right)\)