Difficulty scale 1.0 → 5.0

1.0 Secondary School
Standard secondary school level

General algebra
Ratios, powers, basic factorizations
Plane geometry

1.5 Secondary Olympiad
Lower intermediate / local competitions

Simple combinatorics
Multi-step reasoning
Introductory inequalities
Basic trigonometry

2.0 Regional
Pre-contest / local competitions

Problems needing non-obvious ideas
Structured geometry (circles, centers)
Basic modular arithmetic
Classical combinatorics

2.5 Low National
High pre-contest / easy national contest

Requires a trick
Multi-step complex solutions
Deep trigonometric identities
Elegant combinatorial reasoning

3.0 National
National contest level

Hard geometry
Strong number theory
Clever counting
Advanced transformations

3.5 IMO Easy
IMO easy / strong national level

Long problems with multiple ideas
High creativity
Use of non-trivial results

4.0 IMO / University
IMO / university standard

Very creative insight problems
Complex number theory and combinatorics
High-level geometry
Requires technique + vision

4.5 Graduate
Graduate (Master / early PhD) level

Advanced analysis, algebraic structures, topology, measure-probability
Problems requiring graduate coursework background

7.0 Research
Research / paper-level problems

Problems tied to current research or requiring reading articles
Tools: cohomology, ergodic theory, advanced algebraic geometry, etc.