a:b = c:x

Ratios and fractions have a lot in common.  so you deal with them very similarly.

a:b = c:x

$$\\\frac{a}{b}=\frac{c}{x}\\\\ \frac{b}{a}=\frac{x}{c}\qquad \mbox{Here i turned both sides upside down}\\\\ \frac{x}{c}=\frac{b}{a}\qquad \mbox{Here i swapped sides}\\\\ x=\frac{bc}{a}\qquad \mbox{Here I mult both sides by c}\\\\$$

Does that help Dragon?

$$\\\frac{a}{b}=\frac{c}{x}\\\\ \frac{b}{a}=\frac{x}{c}\\\\ \frac{x}{c}=\frac{b}{a}\\\\ x=\frac{bc}{a}\\\\$$

Can you explain this deeply Melody?

Still confused. But thanks.