What is the largest number that can be added to 6.022x10^23 without changing its displayed value?

The Avogadro constant, named after the scientist Amedeo Avogadro, is the number of constituent particles, usually atoms or molecules, that are contained in the amount of substance given by one mole. The answer, in this case, would be 0.0004999 ... * 10^23 = (10^19) * (4.999 ... [any finite number of 9s]).