Simple algorithm for generating random numbers with bigger smaller probability

Question

I'm currently working on a game, with scrollable screen and I need to find a simple algorithm for placing obstacles in the game. I have a gameSpeed, that is increased in time (from 1 to 12, increased by 0.005 each 1/60s) and a range of available positions between 200 and 600 (ints). I'd like to achieve a bigger probability of receiving smaller number when the speed is bigger, but it's my 14th hour straight coding and I cannot come up with anything usable and not overcomplicated. I'd like to minimize Math and random functions so that the rendering loop won't take too long. Any help appreciated !