\(a,1-\left(\dfrac{1}{5}+\dfrac{2}{3}\right)=1-\dfrac{13}{15}=\dfrac{15}{15}-\dfrac{13}{15}=\dfrac{2}{15}\)
\(b,1\dfrac{5}{6}+2\dfrac{1}{3}\times\dfrac{1}{2}=\dfrac{11}{6}+\dfrac{7}{3}\times\dfrac{1}{2}=\dfrac{11}{6}+\dfrac{7}{6}=\dfrac{18}{6}=3\)
\(\dfrac{8}{25}\times\left(4-\dfrac{1}{4}\right)=\dfrac{8}{25}\times\dfrac{3}{4}=\dfrac{6}{25}\)
\(d,1\dfrac{3}{4}\times2\dfrac{2}{5}:1\dfrac{13}{15}=\dfrac{7}{4}\times\dfrac{12}{5}:\dfrac{28}{15}=\dfrac{21}{5}\times\dfrac{15}{28}=\dfrac{9}{4}\)