$\dfrac{3}{4}+\dfrac{1}{5}=\dfrac{15}{20}+\dfrac{4}{20}=\dfrac{19}{20}$
$\dfrac{5}{6}-\dfrac{2}{9}=\dfrac{15}{18}-\dfrac{4}{18}=\dfrac{11}{18}$
$\dfrac{2}{3}+\dfrac{3}{5}=\dfrac{10}{15}+\dfrac{9}{15}=\dfrac{19}{15}$
$\dfrac{6}{7}-\dfrac{1}{2}=\dfrac{12}{14}-\dfrac{7}{14}=\dfrac{5}{14}$