ratio and proportion help

my professor doesn't really believe in teaching... he assigns homework and expects us to figure it out so we can talk about it in the next class.... Not my idea of great teaching. I'm lost on a question. Any help would be appreciated.

The distance necessary to stop a train at a given speed is inversely proportional to its deceleration. If a train traveling at 30mph requires 180ft to stop when decelerating at .18g, what is the stopping distance at the same speed when it is decelerating at .15g?