$a)$

$3\frac{1}{4}-1\frac{1}{2}+2\frac{7}{8}=3\frac{1}{4}-1\frac{2}{4}+2\frac{7}{8}=$ $2\frac{5}{4}-1\frac{2}{4}+2\frac{7}{8}=1\frac{3}{4}+2\frac{7}{8}=1\frac{6}{8}+2\frac{7}{8}=4\frac{5}{8}$