All of the problems in my multi-grade database of 42,000+ problems are of 4 difficulty levels--level 1 being the simplest and level 4 being the most difficult. These particular problems are ALL of the ones at this level in the various NC (meaning NOT COVERED by the Common Core) Domain standards. They are only for your students who enjoy challenges.
At my school there are only a few who do--and these particular problems were developed for this type of student. Nevertheless, the students in my regular math classes use worksheets that have the problems in difficulty order and I notice that more and more of the students really like to go to the "last problem" to test their own mettle. For this particular domain, this is the full list--along with the solutions (when applicable).
[The problems in All WORKSHEETS and PREVIEWS are printed in difficulty (1 [easy], 2 [not so easy], 3 [proficiency] up to 4 [mastery]) order within a skill. The actual difficulty level appears slightly to the left of the DK box of each problem in a very small font.]