# rational manipulation

#### Ryan\$

##### Junior Member
Hi guys, I just want to verify about something:
why it's right that if I want to calculate x over this equation:

a:b
x:c

then to calculate x, I do (a*c)/b , why it's right? it makes no sense for me, any clue why that rule is always right?!

#### Romsek

##### Full Member
$$\displaystyle \dfrac a b = \dfrac x c$$

multiply both sides by $$\displaystyle c$$

$$\displaystyle \dfrac{a c}{b} = \dfrac{c x}{c} = x$$