у) $$6\frac{1}{2} - 3\frac{3}{4}$$

$$6\frac{1}{2} - 3\frac{3}{4} = \frac{13}{2} - \frac{15}{4} = \frac{13 \cdot 2}{2 \cdot 2} - \frac{15}{4} = \frac{26}{4} - \frac{15}{4} = \frac{26-15}{4} = \frac{11}{4} = 2\frac{3}{4}$$ Ответ: $$2\frac{3}{4}$$