Explanation

This question is prone to algebra errors and it has friendly answer choices, so I'll prefer to analyze it by cases, using the answer choices. I'll start with (A), 10, since it's easy to work with, and then I can double it to get a sense of (D) and the answer choices in between. If the answer is 10, 10 is the number of seminars. In that case, there are 4 unconstrained seminars, originally designed for 18 attendees each, and 6 constrained seminars, originally designed for 18 attendees each but constrained to 15 attendees each. The minimum number of attendees that would attend the four unconstrained one is, first, (since the other ones are overful anyway, they have no where else to go) plus the overflow, which in this case must be people per constrained seminar, of which there are 6, giving us







That's supposed to be the minimum number of attendees in the 4 unconstrained sessions, but it's too small, because we are told that number is 93. However, I can see that adding one constrained seminar will increase the overflow by 3, making this sum correct.

The correct answer is (B).