$\text{a)}\frac{2}{3}+\frac{1}{3}=\frac{3}{3}=1$  

$\text{b)}3\frac{5}{8}+\frac{7}{8}=3\frac{12}{8}=4\frac{4}{8}=4\frac{1}{2}$  

$\text{c)}4\frac{2}{5}+3\frac{4}{5}=7\frac{6}{5}=8\frac{1}{5}$  

$\text{d)}6\frac{2}{5}+3\frac{1}{5}+2\frac{4}{5}=11\frac{7}{5}=12\frac{2}{5}$