a)
\(\dfrac{2}{5}+\dfrac{3}{4}-\dfrac{1}{2}=\dfrac{23}{20}-\dfrac{1}{2}=\dfrac{13}{20}\)
c)
\(\dfrac{1}{7}\cdot\dfrac{2}{5}+\dfrac{1}{5}\cdot\dfrac{1}{7}+\dfrac{1}{7}\cdot\dfrac{4}{5}=\dfrac{1}{7}\cdot\left(\dfrac{2}{5}+\dfrac{1}{5}+\dfrac{4}{5}\right)=\dfrac{1}{7}\cdot\dfrac{7}{5}=\dfrac{1}{5}\)