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.
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