Partial fraction decomposition tells us that we can write this faction as the sum of two fractions (we decompose the given fraction):

\begin{equation*} \frac{1}{(1-x)(1-2x)} = \frac{a}{1-x} + \frac{b}{1-2x} \text{ ~~ for some constants } a \text{ and } b\text{.} \end{equation*}