a)

$\frac{2}{3}+\frac{1}{2}\cdot 3=\frac{2}{3}+\frac{3}{2}=\frac{4}{6}+\frac{9}{6}=\frac{13}{6}=2\frac{1}{6}$

b)

$3\frac{1}{5}-4:8=3\frac{1}{5}-\frac{1}{2}=3\frac{2}{10}-\frac{5}{10}=2\frac{12}{10}-\frac{5}{10}=2\frac{7}{10}$