Bài giải :

$B=\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{1}{120}$

$B=\frac{2}{20}+\frac{2}{30}+\frac{2}{42}+...+\frac{2}{240}$

$B=\frac{2}{4.5}+\frac{2}{5.6}+\frac{2}{6.7}+...+\frac{2}{15.16}$

$B=2\left( \frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{15}-\frac{1}{16} \right)$

$B=2\left( \frac{1}{4}-\frac{1}{16} \right)$

$B=\frac{3}{8}$