This question is absolute garbage. The assumptions given are nowhere near enough to be able to calculate what they are asking. The sample solution is a very rough estimate  like, 10,000 miles away estimate. The best use I can think of for doing a calculation this way is as a check to see if your real work is the right order of magnitude.
From exam day:
Quote:
Originally Posted by Sredni Vashtar
OH finally dumb question what was with the 2nd APBO question in the morning?
Something about the plan changing, so prior service I assume?
But I couldn't think of any way to calculate the new APBO given the like 3 assumptions they offered.

Yep.
Since the actual exam is likely to have some trash questions on it (they always do), I think it's a good idea to work through the model solution on this one to get an idea of one way to bullshit your way to an answer with severely limited information.
My plan for the exam when I don't think the question makes sense is to state what's going on what what assumptions I'm making explicitly.
"The only way I can do this problem is to assume that the retirees are all exactly 65, the 6064 Actives are all exactly 60, and the current Active APBO is distributed equally among all actives. These assumptions don't make much sense but it's the only thing I can do with the information given."
At the same time not spending 15 minutes writing a long essay about it, because it's only a 3 point question.