I just looked at it.
It appears they just assume that you should assume that some of the return air bypasses the coil. Kind of silly. What if the duct was not very well insulated and it gained all that heat from the surrounding air (ie, no return air bypassed the coil)?
Its just not a very well worded question.
In the solutions book, it even throws in that there is 2000 CFM of outside air. That's just a distractor and has no relvance to the problem.
All they want is for you to analyze how the return air that bypasses the coil (again, we just need to totally guess that's whats happening) mixes with the return and outside air that has gone through the coil.
Weird problem. Awkward wording.
Ed