$\text{a)}$

$4\frac{2}{7}+3\frac{5}{7}=4+3+\frac{2}{7}+\frac{5}{7}=7+\frac{7}{7}=7+1=8$

$3\frac{5}{9}+7\frac{8}{9}=3+7+\frac{5}{9}+\frac{8}{9}=10+\frac{13}{9}=10\frac{13}{9}=11\frac{4}{9}$

 

$\text{b)}$

$5-\frac{4}{9}=4\frac{9}{9}-\frac{4}{9}=4\frac{5}{9}$

$8-2\frac{1}{5}=7\frac{5}{5}-2\frac{1}{5}=7-2+\frac{5}{5}-\frac{1}{5}=5\frac{4}{5}$

 

$\text{c)}$

$4\frac{3}{4}-\frac{1}{2}=4\frac{3}{4}-\frac{2}{4}=4\frac{1}{4}$

$6\frac{3}{4}+3\frac{1}{2}=6+3+\frac{3}{4}+\frac{1}{2}=9+\frac{3}{4}+\frac{1}{2}=9+\frac{3}{4}+\frac{2}{4}=9+\frac{5}{4}=10\frac{1}{4}$