For example a water-type Pokemon uses hydro pump under rain conditions.
Will hydro pump's power be x+x/2+x/2 (x is hydro pump's original power), and so 240
or, x 1.5 1.5 and so 270?
I'm not sure what you mean, but Hydro Pump is boosted by 50% in the rain, so 120 + 60 = 180. With STAB, that's 180 + 90 = 270. So it would be the second one, 270