1) Выполним действия:
$$ \frac{3,4 : 1,8 - 1\frac{2}{3} + \frac{7}{9}}{0,918 : 0,51 + 0,45} = \frac{\frac{34}{10} : \frac{18}{10} - \frac{5}{3} + \frac{7}{9}}{\frac{918}{1000} : \frac{51}{100} + \frac{45}{100}} = \frac{\frac{34}{10} \cdot \frac{10}{18} - \frac{5}{3} + \frac{7}{9}}{\frac{918}{1000} \cdot \frac{100}{51} + \frac{45}{100}} = \frac{\frac{17}{9} - \frac{5}{3} + \frac{7}{9}}{\frac{153}{85} + \frac{45}{100}} = \frac{\frac{17}{9} - \frac{15}{9} + \frac{7}{9}}{\frac{3060 + 153 \cdot 9}{1700}} = \frac{\frac{9}{9}}{\frac{3060 + 1377}{1700}} = \frac{1}{\frac{4437}{1700}} = \frac{1700}{4437} \approx 0,38$$
Ответ: $$\frac{1700}{4437} \approx 0,38$$