We have 13 black balls, 3 red balls and 16 white balls.
We want to calculate the probability for getting 16 balls where we want only the white and red balls i.e. zero black balls.
We draw without replacement.
Here is the rule: If we get a white ball, that's fine but if we get a red ball we destroy a white ball (the white one we haven't picked up yet).
So how can we calculate the probability that at least 16 balls are either white or red when we make 20 draws?