How does Vin/R1 * R2 rearrange to Vin * R2/R1? I've recently gone back to uni and after a 10 year gap I'm finding basic working a challenge to remember. In a lecture slide there is a derivation of the non-inverting op-amp formula and it skips through rearranging $$\frac{Vin}{R1}\centerdot R2$$ to $$Vin\centerdot \frac{R2}{R1}$$ It's in the slides and I have cross checked the formula so I know it to be true, but I can't remember or figure out what steps are used to rearrange it.

Order of multiplication doesn't matter

$$ \begin{align} Vin \cdot \frac{1}{R1} \cdot R2 &= \left( Vin \cdot \frac{1}{R1}\right) \cdot R2 = \frac{Vin}{R1} \cdot {R2} \\\ &= Vin \cdot \left( \frac{1}{R1} \cdot R2 \right) = Vin \cdot \frac{R2}{R1} \end{align} $$

The parentheses just note which operation I do first. So the 'fraction' part can be under any term in the multiplication.