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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.