a) 0,(18) = x
100x = 18,(18)
100x - x = 18,(18) - 0,(18)
99x = 18
$$x = \frac{18}{99} = \frac{2}{11}$$
б) 0,0(24) = x
10x = 0,(24)
1000x = 24,(24)
1000x - 10x = 24,(24) - 0,(24)
990x = 24
$$x = \frac{24}{990} = \frac{4}{165}$$
Ответ: а) $$\frac{2}{11}$$; б) $$\frac{4}{165}$$