How many states require motorcycle riders to wear helmets?

According to AP writer, Jennifer C. Yates in the Hanover, PA <i><A HREF="http://www.eveningsun.com/ci_9574790">Evening Sun</A></i>, "Twenty states have laws requiring all motorcycle riders to wear helmets, while 27 states have laws that require minors or passengers to wear head protection. Iowa, Illinois and New Hampshire have no helmet laws."