Ok lets see if I got this.
Lets say I have 200 boxes.
3/4 of the boxes have something in them.
I want to find how many empty boxes there are
so i split the boxes into 2 groups

And I count untill I found an empty box lets say the empty box is the 10th. So then i go directly to 20 if that box is empty i go to box 40 and so on untill i hit 200 but if box 20 is empty i have to check every box from 10 to 20 to see if there are any other empty ones?