its a trick question.

the answer is 4 hours.
it asks for the LEAST amount of time B must operate.
Therefore we need to find how much A can produce by itself in 8 hours.
= 9k * 8 =72k

100k is our goal, so b needs to produce 100k-72k = 28k

b produces 7k per hour => 28k/7k = 4 hours.

p.s.
What did you get on this practice test? (I remember I also had this as the last question in my GMATprep test 2 and I ended with a 48 raw on that test I believe)

I got a 40, finishing with this question answered wrong. I have 37 seconds, so I just hit whatever was closest. To be fair, I couldn't figure out how to solve this problem after the test. I ended up with pretty big numbers something like 3 and 15/16 and decided from then that I was not heading in the right direction.