$a)1\frac{7}{10}+1\frac{1}{15}:2\frac{2}{3}=1\frac{7}{10}+\frac{16}{15}:\frac{8}{3}=1\frac{7}{10}+\frac{{\sout{16}}^2}{15}\cdot \frac{3}{{\sout{8}}^1}=1\frac{7}{10}+\frac{6}{15}=1\frac{7}{10}+\frac{2}{5}=1\frac{7}{10}+\frac{4}{10}=1\frac{11}{10}=2\frac{1}{10}$

$b)6\frac{2}{5}-3^2:1\frac{1}{2}=6\frac{2}{5}-9:\frac{3}{2}=6\frac{2}{5}-{\sout{9}}^3\cdot \frac{2}{{\sout{3}}^1}=6\frac{2}{5}-6=\frac{2}{5}$