Whenever authority or direction is given by statute to a county containing a city having a population of more than seventy thousand according to the official United States census or the latest United States census, or words of similar import, such legislation shall be fully applicable to a county containing a city having a population of more than seventy thousand as revealed by a determination of the population of areas annexed to such city after the date of a decennial census, where such determination of population is certified by the Bureau of the Census, by adding the certified population of such annexed areas to the officially certified population of such city.