$a)\left(53,1-52\frac{2}{50}\right)\cdot \frac{10}{21}=1,05\cdot \frac{10}{21}=\frac{{\sout{105}}^1}{{\sout{100}}^2}\cdot \frac{{\sout{10}}^1}{{\sout{21}}^1}=\frac{1}{2}$

$b_{13}\frac{1}{4}+1\frac{2}{5}+5,685=13,25+1,4+5,685=20,335=20\frac{335}{1000}=20\frac{67}{200}$