7) $$\begin{array}{rcl}7\frac{11}{21}+5\frac{8}{21}+2\frac{10}{21}&=&12\frac{19}{21}+2\frac{10}{21}=14\frac{29}{21}=15\frac{8}{21}\end{array}$$
Ответ: $$15\frac{8}{21}$$