$\text{a)}4,2-\frac{2}{5}=4,2-\frac{4}{10}=4,2-0,4=3,8$

$5\frac{2}{5}-1,25=5\frac{4}{10}-1,25=5,4-1,25=4,15$

$0,9-\frac{3}{8}=\frac{9}{10}-\frac{3}{8}=\frac{36}{40}-\frac{15}{40}=\frac{21}{40}$

 

 

$\text{b)}3,75-1\frac{1}{6}=3\frac{75}{100}-1\frac{1}{6}=3\frac{3}{4}-1\frac{1}{6}=3\frac{9}{12}-1\frac{2}{12}=2\frac{7}{12}$  

$4\frac{2}{3}-1,5=4\frac{2}{3}-1\frac{5}{10}=4\frac{2}{3}-1\frac{1}{2}=4\frac{4}{6}-1\frac{3}{6}=3\frac{1}{6}$  

$6\frac{1}{7}-2\frac{2}{5}=6\frac{5}{35}-2\frac{14}{35}=5\frac{40}{35}-2\frac{14}{35}=3\frac{26}{35}$