Tim, Like aairly, I too was thrown off by the Sister constraint, which by your careful reasoning, amounts to a red herring. I find nothing wrong with your response to aairly, but the "Show Explanation" for this problem clearly states that one of the constraints is "Minimum of 3 art or...

I have not seen this problem yet, but could it be that the explanation says that sister still needs to add a "minimum of 3 art or architecture activities" since she only has one on the schedule now and needs 4? Again, I haven't seen the problem but I'm speculating since that seems like an...