How can complex fractions be used to solve problems involving ratios

When you have ratios, and some unknowns, you can create complex fractions from them. Bring them to the same denominator, and solve for x.

Example - we have this proportion:
2-x
5-45
And we can change it into fraction:
\frac{2}{5}=\frac{x}{45}

\frac{2*9}{5*9}=\frac{x}{45}

\frac{18}{45}=\frac{x}{45}
18=x

In case of more complex fractions it may come in handy.

The answer would be: b