80. Найдите, при каком значении z верно равенство: а) \frac{25}{45} = \frac{z}{9}; б) \frac{z}{7} = \frac{36}{42}; в) \frac{39}{78} = \frac{3}{z}; г) \frac{7}{z} = \frac{35}{40}.

а) \frac{25}{45} = \frac{z}{9} => z = \frac{25 * 9}{45} = \frac{25}{5} = 5 б) \frac{z}{7} = \frac{36}{42} => z = \frac{36 * 7}{42} = \frac{36}{6} = 6 в) \frac{39}{78} = \frac{3}{z} => z = \frac{3 * 78}{39} = 3 * 2 = 6 г) \frac{7}{z} = \frac{35}{40} => z = \frac{7 * 40}{35} = \frac{40}{5} = 8