wszędzie sprowadzasz do wspólnego mianownika
[tex]\frac{8}{9} - \frac{7}{15} = \frac{40}{45} - \frac{21}{45} = \frac{19}{45} \\\\\frac{1}{6} + \frac{5}{8} = \frac{4}{24} + \frac{15}{24} = \frac{19}{24} \\\\\frac{17}{20} - \frac{8}{15} = \frac{51}{60} - \frac{32}{60} = \frac{19}{60}[/tex]
