Domino Skyscraper Math Question

We are trying to immerse ourselves in problem solving processes during the last weeks of math this year.  Today we looked at the following problem (found here  http://www.101qs.com/1989-domino-skyscraper ).  We asked how many dominoes would it take to knock over a domino the height of a skyscraper?  We made use of the graphic below to help us decide on a skyscraper height of 300m (between the tallest, and the smallest average height).