13) $$\frac{1}{4} - \frac{3}{25}$$; 14) $$\frac{1}{5} - \frac{27}{50}$$; 15) $$\frac{1}{2} - \frac{9}{25}$$; 16) $$\frac{1}{5} - \frac{3}{4}$$; 17) $$\frac{1}{2} - \frac{13}{50}$$

13) $$\frac{1}{4} - \frac{3}{25} = \frac{1 \cdot 25}{4 \cdot 25} - \frac{3 \cdot 4}{25 \cdot 4} = \frac{25}{100} - \frac{12}{100} = \frac{25 - 12}{100} = \frac{13}{100}$$ 14) $$\frac{1}{5} - \frac{27}{50} = \frac{1 \cdot 10}{5 \cdot 10} - \frac{27}{50} = \frac{10}{50} - \frac{27}{50} = \frac{10 - 27}{50} = \frac{-17}{50} = -\frac{17}{50}$$ 15) $$\frac{1}{2} - \frac{9}{25} = \frac{1 \cdot 25}{2 \cdot 25} - \frac{9 \cdot 2}{25 \cdot 2} = \frac{25}{50} - \frac{18}{50} = \frac{25 - 18}{50} = \frac{7}{50}$$ 16) $$\frac{1}{5} - \frac{3}{4} = \frac{1 \cdot 4}{5 \cdot 4} - \frac{3 \cdot 5}{4 \cdot 5} = \frac{4}{20} - \frac{15}{20} = \frac{4 - 15}{20} = \frac{-11}{20} = -\frac{11}{20}$$ 17) $$\frac{1}{2} - \frac{13}{50} = \frac{1 \cdot 25}{2 \cdot 25} - \frac{13}{50} = \frac{25}{50} - \frac{13}{50} = \frac{25 - 13}{50} = \frac{12}{50} = \frac{6}{25}$$ Ответы: $$\frac{13}{100}$$, $$\frac{-17}{50}$$, $$\frac{7}{50}$$, $$\frac{-11}{20}$$, $$\frac{6}{25}$$