Math is cumulative. If a student cheats through Level G, they will be completely lost when they reach Level H (Linear Equations) and Level I (Factorization).
Often referred to as the "make or break" stage, Kumon G Level (typically aligned with middle school algebra for Math and advanced critical reading for Reading) represents a massive jump in difficulty. It is at this stage that a specific search term becomes incredibly popular among stressed students and parents: kumon g level answer book