I wasn't responding to your Post #12 or #18. But now that you went out of the way to put them on the table, neither of those posts pointed out, explicitly, that Zulfi doesn't need the classic one equation per unknown because he is not solving for all of the unknowns. That point is implicit in your recommendations, and hence easily overlooked. So Zulfi could very easily end up thinking that what you've recommended is how to get that "third equation" he is looking for, when that's not what you were doing.Did you not read my post #12 and #18?
I also recommended that Zulfi start things off by clearly defining what he is trying to solve for. You did essentially the same thing, but in a slightly indirect faction. I chose to do it is a very direct fashion. I could have (perhaps should have) done it even more direct.
Fraction of students not going on trip = (students not going on trip) / (number of students)
That's a clear statement of what we want to find.
Then I could have illustrated the next step in the process of moving from what we want to what we have to work with:
students not going on trip = (juniors not going) + (seniors not going)
number of students = (juniors) + (seniors)
The next step would be to utilize the information given to move these equations closer to something that can be evaluated.