Bài giải :

$B=\left( 1-\frac{1}{3} \right)\left( 1-\frac{1}{6} \right)\left( 1-\frac{1}{10} \right)\left( 1-\frac{1}{15} \right)\left( 1-\frac{1}{21} \right)\left( 1-\frac{1}{28} \right)$

$B=\frac{2}{3}.\frac{5}{6}.\frac{9}{10}.\frac{14}{15}.\frac{20}{21}.\frac{27}{28}=\frac{{{2.5.3}^{2}}{{.2.7.2}^{2}}{{.5.3}^{3}}}{{{3.2.3.2.5.3.5.3.7.2}^{2}}.7}$

$B=\frac{{{2}^{4}}{{.3}^{5}}{{.5}^{2}}.7}{{{2}^{4}}{{.3}^{4}}{{.5}^{2}}{{.7}^{2}}}=\frac{3}{7}$