a)

$\frac{1}{7}+\frac{2}{7}+\frac{3}{7}+\frac{4}{7}+\frac{5}{7}+\frac{6}{7}+\frac{7}{7}+\frac{8}{7}+\frac{9}{7}+\frac{10}{7}=\frac{6}{7}+\frac{9}{7}+\frac{13}{7}+\frac{17}{7}+\frac{10}{7}=$   $=\frac{15}{7}+\frac{30}{7}+\frac{10}{7}=\frac{15}{7}+\frac{40}{7}=\frac{55}{7}=7\frac{6}{7}$