a)

$4-2\frac{2}{5}=\left(4+\frac{3}{5}\right)-\left(2\frac{2}{5}+\frac{3}{5}\right)=4\frac{3}{5}-3=1\frac{3}{5}$  

 

$2-1\frac{5}{8}=\left(2+\frac{3}{8}\right)-\left(1\frac{5}{8}+\frac{3}{8}\right)=2\frac{3}{8}-2=\frac{3}{8}$  

 

$5-3\frac{1}{7}=\left(5+\frac{6}{7}\right)-\left(3\frac{1}{7}+\frac{6}{7}\right)=5\frac{6}{7}-4=1\frac{6}{7}$  

 

 

b)

$3\frac{2}{9}-1\frac{5}{6}=3\frac{4}{18}-1\frac{15}{18}=\left(3\frac{4}{18}+\frac{3}{18}\right)-\left(1\frac{15}{18}+\frac{3}{18}\right)=3\frac{7}{18}-2=1\frac{7}{18}$  

 

$5\frac{2}{7}-2\frac{3}{4}=5\frac{8}{28}-2\frac{21}{28}=\left(5\frac{8}{28}+\frac{7}{28}\right)-\left(2\frac{21}{28}+\frac{7}{28}\right)=5\frac{15}{28}-3=2\frac{15}{28}$  

 

$2\frac{1}{3}-1\frac{5}{7}=2\frac{7}{21}-1\frac{15}{21}=\left(2\frac{7}{21}+\frac{6}{21}\right)-\left(1\frac{15}{21}+\frac{6}{21}\right)=2\frac{13}{21}-2=\frac{13}{21}$