Ответ:

а) 9/10

б) 9/2

и) 7/3

Объяснение:

[tex]\displaystyle \frac{a}{b} = 0.9=\frac{9}{10} \\\\\\[/tex]

a) [tex]\displaystyle \frac{b}{a} =\frac{10}{9}[/tex]

б)  [tex]\displaystyle \frac{3a}{1.8b} =\frac{3*9}{1.8*10}=\frac{27}{18} =\frac{9}{2}[/tex]

в) [tex]\displaystyle \frac{3a+b}{2a-3b} =\frac{2*9+10}{2*9-3*10}=\frac{28}{12} =\frac{7}{3}[/tex]