1) \(18 \cdot (\frac{1}{3} - \frac{1}{9}) = 18 \cdot \frac{1}{3} - 18 \cdot \frac{1}{9} = 6 - 2 = 4\)
2) \((2 - \frac{3}{4} - \frac{4}{5}) \cdot 20 = 2 \cdot 20 - \frac{3}{4} \cdot 20 - \frac{4}{5} \cdot 20 = 40 - 15 - 16 = 9\)
3) \((\frac{1}{3} - \frac{1}{6} - \frac{1}{9}) \cdot 18 = \frac{1}{3} \cdot 18 - \frac{1}{6} \cdot 18 - \frac{1}{9} \cdot 18 = 6 - 3 - 2 = 1\)
4) \((\frac{5}{6} + \frac{5}{9}) \cdot \frac{18}{25} = \frac{5}{6} \cdot \frac{18}{25} + \frac{5}{9} \cdot \frac{18}{25} = \frac{3}{5} + \frac{2}{5} = \frac{5}{5} = 1\)