Eggs are EITHER very hard OR very fragile. This is the trick in the question: There is no continuous distribution for the egg fragility between the two extremes, it is two distinct and extreme possibilities ONLY.
Hence you need only ONE drop. It can be from ANY story of the building.
By doing that, you would certainly know which egg you have. (If it breaks, you have the soft one, if it survives, you would have the strong one.)
It is that simple.