a) $\frac{8}{11}−1\frac{1}{4}+\frac{3}{14}−(\frac{−13}{21})$+0,25=$\frac{8}{21}+\frac{13}{21}−\frac{5}{4}+\frac{1}{4}+\frac{3}{14}=\frac{3}{14}$

b) $\frac{5}{8}.2\frac{2}{5}−\frac{5}{8}.1\frac{1}{3}=\frac{5}{8}.(\frac{12}{5}−\frac{4}{3})=\frac{5}{8}.\frac{16}{15}=\frac{2}{3}$

c) $(\frac{4}{9}−\frac{3}{5}):1\frac{1}{5}+(\frac{5}{9}+\frac{1}{5}):1\frac{1}{5}=(\frac{4}{9}−\frac{3}{5}+\frac{5}{9}+\frac{1}{5}):\frac{6}{5}=(1−\frac{2}{5}).\frac{5}{6}=\frac{1}{2}$

d) $\frac{6}{7}:(\frac{3}{26}−\frac{3}{13})+\frac{6}{7}:(\frac{1}{10}−\frac{8}{5})=\frac{6}{7}:(\frac{3}{26}−\frac{6}{26})+\frac{6}{7}:(\frac{1}{10}−\frac{16}{10})=\frac{6}{7}.[(\frac{−26}{3})+(\frac{−2}{3})]$=−8