7) 3/8 + 3/14 - 1/28

$$\frac{3}{8} + \frac{3}{14} - \frac{1}{28} = \frac{3*49}{8*49} + \frac{3*28}{14*28} - \frac{1*8}{28*8} = \frac{147}{392} + \frac{84}{392} - \frac{8}{224} = \frac{147 + 84 - 8}{392} = \frac{223}{392}$$
Ответ: $$\frac{223}{392}$$