21. Вычислите: 5/4 + (−7 3/4 + 7/4 · 2 2/7) : 5/9.

\[\frac74\cdot2\frac27=\frac74\cdot\frac{16}{7}=4.\]

\[-7\frac34+4=-\frac{31}{4}+\frac{16}{4}=-\frac{15}{4}.\]

\[-\frac{15}{4}:\frac59=-\frac{15}{4}\cdot\frac95=-\frac{27}{4}.\]

\[\frac54-\frac{27}{4}=-\frac{22}{4}=-\frac{11}{2}.\]

Ответ: \(-\frac{11}{2}\).