This question was actually asked by google employers to check the thought process of the employee. It is to see how well employee can do “guestimate” that is what is referred for such question. It is a little tricky but if you good at numbers and height width calculation, you will surely come up with some answer. Anyway the answer to this question was something 750,000 tons or 1.5 billion pound of weight, after considering all the parameters like single floor widthXheight, weight, etc.   _{Your Comment}