In the form: .abbbb...

For a decimal with a non-repeating part followed by a repeating digit, like $.abbbb...$, the fractional form is $\frac{ab - a}{90}$, where $ab$ is the two-digit number formed by the first two digits. This can be derived by treating the decimal as $\frac{a}{10} + \frac{b}{90}$. For example, $.27777... = \frac{27-2}{90} = \frac{25}{90} = \frac{5}{18}$.