Concrete Operational v. Formal Operational

Each day, I spend quite a bit of time thinking about which problems in the book are unneccessary for students to do for homework. Because of this, I end up with problem sets that look like this:

p.150-151

#2-5all

#10-14even

#18-24all

#27, 30

I spend quite a bit of time on this, so it's frustrating that students complain that there's too much homework, when I went through the trouble of making sure there were actually fewer problems in the homework set.

The thing is, when I give them problem sets that look like this:

p. 150-151 #2-30, 31

They don't complain as much, despite the fact that there are exactly the same number of problems.

I was thinking about this and about achievement in my class, and some of my Psych background popped back into my head. Could it be that most of my students are not yet in the formal operational stage of development? Or, more importantly, will they ever reach that stage?

Anyways, not a fully formed thought, but I better get back to lesson planning...grading actually.