Set bullet speed according to variable

Hi! i need some help!

i want to make the bullet speed to increase every x seconds,

i tryed to create a var named saw_speed (that's the instance who needs to speed up)but i did'nt figure out how to associate with the bullet speed

if i could put like this:

every x seconds set bullet speed +100,

would be nice if works that way, but i m not able to set anything but a specific number.

if somebody could tell me what to do, i will appreciate.

I would use a global variable, like this:

<img src="http://infinitepossibilitygames.com/demos/BulletSpeed/bulletspeedincrease.png" border="0" />

thank you! Theubie, that solve my problem.