From: United States
Look at this 4 and 5. There are 6 boxes that overlap the 4 and the 5. The 4 needs 3 more to complete it. Since the overlap can't contain more than 3, the 5 needs 2 more to complete it. Since there are only 2 more boxes possible to fill for the 5, the 2 boxes to the left of the 5 are solid. Hope that makes sense. It's much easier for me to understand than to explain.