B = 1/5 + 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10
B = 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10
B = 1/5 + 1/5 - 1/10
B = 2/5 - 1/10
B = 3/10
\(B=\frac{1}{5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{9.10}\)
\(=\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{9}-\frac{1}{10}\)
\(=\frac{1}{5}+\frac{1}{5}-\frac{1}{10}=\frac{4}{10}-\frac{1}{10}=\frac{3}{10}\)