Solve the equation for n, the number of molecules. PV=nRT

PV/RT=n

In order to isolate n, which I'm assuming you're trying to do, you have to seperate it from RT. Since you're multiplying the two, you must do the inverse operation, in this case, division. So, divide RT to both sides, and voila, you have the equation that was aforementioned.

N=PV/RT because you want to bring RT to the other side