Sorry I was not more specific and actually you need two variable's. A object variable and a global.
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(subevent) object variable greater than 0--->subtract 1 from object variable
(subevent) object variable =0------->spawn bullet
------->set object variable to global variable
on collision between player and powerup---->destroy powerup
----> subtract from global variable
---->set object variable to global variable
Play around with your initial settings , set your object and global variable's the same initially and subtract from your global an amount that gives you the speed increase that you want , remember 60 ticks per second or what ever your fps is. So if you start with 60 and then subtract 10 every time you hit a powerup your spawn rate will increase as per powerup's. You might want to clamp your global variable so as not to go below a certain number . Hope this will help and is more clear !