Sixth grade math. Don't talk about calculation, and don't talk about application. Graphics or concepts are preferred. For example, the understanding of reciprocal, the understanding of circle, the significance and basic nature of the proportion of senior students, etc. The understanding of angles, the understanding and characteristics of triangles and quadrangles, and the combination of graphics in the lower grades can all include the understanding of some large numbers and the understanding of numbers within 1000 and 10,000.