Yes, this is a typo, they meant a-b, obviously such a blunder on a question while taking a practice test can throw off any student. I actually don't like this question because the example they use a=11 and b=7 does not eliminate answer choice D which is equal to a+b-1 or 11+7-1=17 which is a prime number. This is also the case with choice E, where 11+7=18 is a multiple of 3. I would advise you to stick to official GMAT questions, there is plenty of material out there. There is no need to spend your valuable time on poorly written questions.
Free Video Explanations: OFFICIAL GUIDE FOR GMAT REVIEW 2015.