It also depends on how you claim marital status and dependents. That does not have to match when you file for taxes. IE: You can claim 0 dependents and really have 5, then you will get a nice check from the government when you file, or you can claim 9 and have less withheld, but you will owe a lot ...

Plus it depends if any is withheld for other reasons, such as medical insurance.

If I had to guess, it will be about $5/hour taken out of the $15 (leaving about $10/hour), but it depends on so many factors... IE: If part time will be less taken out than if full time.