Those of you that came up with 600 are correct.
When you subtract the number of people wearing 1 earing (30) from the total you end up with 570 left. of those 1/2 (285) are wearing no earings (0) and the other half are wearing 2 earings (2x285=570) added with the 30 that wear 1 earing you get 600.
basicly the people wearing 2 earings make up for the people wearing 1 earing.
