From what I have seen it is closer to 20%
As far as how the polls are conducted - it varies from organization to organization - and how the question is phrased. But phrasing of questions are more likely to effect how someone answers questions like - “are you for the death tax?” vs “are you for the estate tax?” vs “do you think it is fair to be taxed when you die?”
Whether someone thinks Obama is a Muslim or not - I think the wording wouldn’t matter that much.
One in five doesn’t surprise me at all - I know people that believe this - I know people that think he is “the Antichrist”. When you really think about it - the fact that we elected a black man named Barack Hussein Obama is amazing (based on the irrational fears of a large percentage of Americans). Keep in mind 33% don’t know Romney is a Mormon and 44% say they don’t know Obama’s religion - some people really don’t follow the news.