Millisecond Forums

I am trying to modify the Bart script(found at https://www.millisecond.com/download/library/bart/) by having the screen flash red in the init trial. I would like this to happen randomly only in let's say 20 of the 30 times the trial runs. Question is how does one make it so that it does not flash every time, rather a set number of times, and not necessarily all in a row as well? What I have added so far is a simple background shape which flashes in /ontrialbegin in the init trial